Merge branch 'master' of github.com:alshopov/git-po

[git.git] / fsmonitor.c

#define USE_THE_REPOSITORY_VARIABLE

#include "git-compat-util.h"
#include "config.h"
#include "dir.h"
#include "environment.h"
#include "ewah/ewok.h"
#include "fsmonitor.h"
#include "fsmonitor-ipc.h"
#include "name-hash.h"
#include "repository.h"
#include "run-command.h"
#include "strbuf.h"
#include "trace2.h"

#define INDEX_EXTENSION_VERSION1        (1)
#define INDEX_EXTENSION_VERSION2        (2)
#define HOOK_INTERFACE_VERSION1         (1)
#define HOOK_INTERFACE_VERSION2         (2)

struct trace_key trace_fsmonitor = TRACE_KEY_INIT(FSMONITOR);

static void assert_index_minimum(struct index_state *istate, size_t pos)
{
        if (pos > istate->cache_nr)
                BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" > %u)",
                    (uintmax_t)pos, istate->cache_nr);
}

static void fsmonitor_ewah_callback(size_t pos, void *is)
{
        struct index_state *istate = (struct index_state *)is;
        struct cache_entry *ce;

        assert_index_minimum(istate, pos + 1);

        ce = istate->cache[pos];
        ce->ce_flags &= ~CE_FSMONITOR_VALID;
}

static int fsmonitor_hook_version(void)
{
        int hook_version;

        if (git_config_get_int("core.fsmonitorhookversion", &hook_version))
                return -1;

        if (hook_version == HOOK_INTERFACE_VERSION1 ||
            hook_version == HOOK_INTERFACE_VERSION2)
                return hook_version;

        warning("Invalid hook version '%i' in core.fsmonitorhookversion. "
                "Must be 1 or 2.", hook_version);
        return -1;
}

int read_fsmonitor_extension(struct index_state *istate, const void *data,
        unsigned long sz)
{
        const char *index = data;
        uint32_t hdr_version;
        uint32_t ewah_size;
        struct ewah_bitmap *fsmonitor_dirty;
        int ret;
        uint64_t timestamp;
        struct strbuf last_update = STRBUF_INIT;

        if (sz < sizeof(uint32_t) + 1 + sizeof(uint32_t))
                return error("corrupt fsmonitor extension (too short)");

        hdr_version = get_be32(index);
        index += sizeof(uint32_t);
        if (hdr_version == INDEX_EXTENSION_VERSION1) {
                timestamp = get_be64(index);
                strbuf_addf(&last_update, "%"PRIu64"", timestamp);
                index += sizeof(uint64_t);
        } else if (hdr_version == INDEX_EXTENSION_VERSION2) {
                strbuf_addstr(&last_update, index);
                index += last_update.len + 1;
        } else {
                return error("bad fsmonitor version %d", hdr_version);
        }

        istate->fsmonitor_last_update = strbuf_detach(&last_update, NULL);

        ewah_size = get_be32(index);
        index += sizeof(uint32_t);

        fsmonitor_dirty = ewah_new();
        ret = ewah_read_mmap(fsmonitor_dirty, index, ewah_size);
        if (ret != ewah_size) {
                ewah_free(fsmonitor_dirty);
                return error("failed to parse ewah bitmap reading fsmonitor index extension");
        }
        istate->fsmonitor_dirty = fsmonitor_dirty;

        if (!istate->split_index)
                assert_index_minimum(istate, istate->fsmonitor_dirty->bit_size);

        trace2_data_string("index", NULL, "extension/fsmn/read/token",
                           istate->fsmonitor_last_update);
        trace_printf_key(&trace_fsmonitor,
                         "read fsmonitor extension successful '%s'",
                         istate->fsmonitor_last_update);
        return 0;
}

void fill_fsmonitor_bitmap(struct index_state *istate)
{
        unsigned int i, skipped = 0;
        istate->fsmonitor_dirty = ewah_new();
        for (i = 0; i < istate->cache_nr; i++) {
                if (istate->cache[i]->ce_flags & CE_REMOVE)
                        skipped++;
                else if (!(istate->cache[i]->ce_flags & CE_FSMONITOR_VALID))
                        ewah_set(istate->fsmonitor_dirty, i - skipped);
        }
}

void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
{
        uint32_t hdr_version;
        uint32_t ewah_start;
        uint32_t ewah_size = 0;
        int fixup = 0;

        if (!istate->split_index)
                assert_index_minimum(istate, istate->fsmonitor_dirty->bit_size);

        put_be32(&hdr_version, INDEX_EXTENSION_VERSION2);
        strbuf_add(sb, &hdr_version, sizeof(uint32_t));

        strbuf_addstr(sb, istate->fsmonitor_last_update);
        strbuf_addch(sb, 0); /* Want to keep a NUL */

        fixup = sb->len;
        strbuf_add(sb, &ewah_size, sizeof(uint32_t)); /* we'll fix this up later */

        ewah_start = sb->len;
        ewah_serialize_strbuf(istate->fsmonitor_dirty, sb);
        ewah_free(istate->fsmonitor_dirty);
        istate->fsmonitor_dirty = NULL;

        /* fix up size field */
        put_be32(&ewah_size, sb->len - ewah_start);
        memcpy(sb->buf + fixup, &ewah_size, sizeof(uint32_t));

        trace2_data_string("index", NULL, "extension/fsmn/write/token",
                           istate->fsmonitor_last_update);
        trace_printf_key(&trace_fsmonitor,
                         "write fsmonitor extension successful '%s'",
                         istate->fsmonitor_last_update);
}

/*
 * Call the query-fsmonitor hook passing the last update token of the saved results.
 */
static int query_fsmonitor_hook(struct repository *r,
                                int version,
                                const char *last_update,
                                struct strbuf *query_result)
{
        struct child_process cp = CHILD_PROCESS_INIT;
        int result;

        if (fsm_settings__get_mode(r) != FSMONITOR_MODE_HOOK)
                return -1;

        strvec_push(&cp.args, fsm_settings__get_hook_path(r));
        strvec_pushf(&cp.args, "%d", version);
        strvec_pushf(&cp.args, "%s", last_update);
        cp.use_shell = 1;
        cp.dir = repo_get_work_tree(the_repository);

        trace2_region_enter("fsm_hook", "query", NULL);

        result = capture_command(&cp, query_result, 1024);

        if (result)
                trace2_data_intmax("fsm_hook", NULL, "query/failed", result);
        else
                trace2_data_intmax("fsm_hook", NULL, "query/response-length",
                                   query_result->len);

        trace2_region_leave("fsm_hook", "query", NULL);

        return result;
}

/*
 * Invalidate the FSM bit on this CE.  This is like mark_fsmonitor_invalid()
 * but we've already handled the untracked-cache, so let's not repeat that
 * work.  This also lets us have a different trace message so that we can
 * see everything that was done as part of the refresh-callback.
 */
static void invalidate_ce_fsm(struct cache_entry *ce)
{
        if (ce->ce_flags & CE_FSMONITOR_VALID) {
                trace_printf_key(&trace_fsmonitor,
                                 "fsmonitor_refresh_callback INV: '%s'",
                                 ce->name);
                ce->ce_flags &= ~CE_FSMONITOR_VALID;
        }
}

static size_t handle_path_with_trailing_slash(
        struct index_state *istate, const char *name, int pos);

/*
 * Use the name-hash to do a case-insensitive cache-entry lookup with
 * the pathname and invalidate the cache-entry.
 *
 * Returns the number of cache-entries that we invalidated.
 */
static size_t handle_using_name_hash_icase(
        struct index_state *istate, const char *name)
{
        struct cache_entry *ce = NULL;

        ce = index_file_exists(istate, name, strlen(name), 1);
        if (!ce)
                return 0;

        /*
         * A case-insensitive search in the name-hash using the
         * observed pathname found a cache-entry, so the observed path
         * is case-incorrect.  Invalidate the cache-entry and use the
         * correct spelling from the cache-entry to invalidate the
         * untracked-cache.  Since we now have sparse-directories in
         * the index, the observed pathname may represent a regular
         * file or a sparse-index directory.
         *
         * Note that we should not have seen FSEvents for a
         * sparse-index directory, but we handle it just in case.
         *
         * Either way, we know that there are not any cache-entries for
         * children inside the cone of the directory, so we don't need to
         * do the usual scan.
         */
        trace_printf_key(&trace_fsmonitor,
                         "fsmonitor_refresh_callback MAP: '%s' '%s'",
                         name, ce->name);

        /*
         * NEEDSWORK: We used the name-hash to find the correct
         * case-spelling of the pathname in the cache-entry[], so
         * technically this is a tracked file or a sparse-directory.
         * It should not have any entries in the untracked-cache, so
         * we should not need to use the case-corrected spelling to
         * invalidate the the untracked-cache.  So we may not need to
         * do this.  For now, I'm going to be conservative and always
         * do it; we can revisit this later.
         */
        untracked_cache_invalidate_trimmed_path(istate, ce->name, 0);

        invalidate_ce_fsm(ce);
        return 1;
}

/*
 * Use the dir-name-hash to find the correct-case spelling of the
 * directory.  Use the canonical spelling to invalidate all of the
 * cache-entries within the matching cone.
 *
 * Returns the number of cache-entries that we invalidated.
 */
static size_t handle_using_dir_name_hash_icase(
        struct index_state *istate, const char *name)
{
        struct strbuf canonical_path = STRBUF_INIT;
        int pos;
        size_t len = strlen(name);
        size_t nr_in_cone;

        if (name[len - 1] == '/')
                len--;

        if (!index_dir_find(istate, name, len, &canonical_path))
                return 0; /* name is untracked */

        if (!memcmp(name, canonical_path.buf, canonical_path.len)) {
                strbuf_release(&canonical_path);
                /*
                 * NEEDSWORK: Our caller already tried an exact match
                 * and failed to find one.  They called us to do an
                 * ICASE match, so we should never get an exact match,
                 * so we could promote this to a BUG() here if we
                 * wanted to.  It doesn't hurt anything to just return
                 * 0 and go on because we should never get here.  Or we
                 * could just get rid of the memcmp() and this "if"
                 * clause completely.
                 */
                BUG("handle_using_dir_name_hash_icase(%s) did not exact match",
                    name);
        }

        trace_printf_key(&trace_fsmonitor,
                         "fsmonitor_refresh_callback MAP: '%s' '%s'",
                         name, canonical_path.buf);

        /*
         * The dir-name-hash only tells us the corrected spelling of
         * the prefix.  We have to use this canonical path to do a
         * lookup in the cache-entry array so that we repeat the
         * original search using the case-corrected spelling.
         */
        strbuf_addch(&canonical_path, '/');
        pos = index_name_pos(istate, canonical_path.buf,
                             canonical_path.len);
        nr_in_cone = handle_path_with_trailing_slash(
                istate, canonical_path.buf, pos);
        strbuf_release(&canonical_path);
        return nr_in_cone;
}

/*
 * The daemon sent an observed pathname without a trailing slash.
 * (This is the normal case.)  We do not know if it is a tracked or
 * untracked file, a sparse-directory, or a populated directory (on a
 * platform such as Windows where FSEvents are not qualified).
 *
 * The pathname contains the observed case reported by the FS. We
 * do not know it is case-correct or -incorrect.
 *
 * Assume it is case-correct and try an exact match.
 *
 * Return the number of cache-entries that we invalidated.
 */
static size_t handle_path_without_trailing_slash(
        struct index_state *istate, const char *name, int pos)
{
        /*
         * Mark the untracked cache dirty for this path (regardless of
         * whether or not we find an exact match for it in the index).
         * Since the path is unqualified (no trailing slash hint in the
         * FSEvent), it may refer to a file or directory. So we should
         * not assume one or the other and should always let the untracked
         * cache decide what needs to invalidated.
         */
        untracked_cache_invalidate_trimmed_path(istate, name, 0);

        if (pos >= 0) {
                /*
                 * An exact match on a tracked file. We assume that we
                 * do not need to scan forward for a sparse-directory
                 * cache-entry with the same pathname, nor for a cone
                 * at that directory. (That is, assume no D/F conflicts.)
                 */
                invalidate_ce_fsm(istate->cache[pos]);
                return 1;
        } else {
                size_t nr_in_cone;
                struct strbuf work_path = STRBUF_INIT;

                /*
                 * The negative "pos" gives us the suggested insertion
                 * point for the pathname (without the trailing slash).
                 * We need to see if there is a directory with that
                 * prefix, but there can be lots of pathnames between
                 * "foo" and "foo/" like "foo-" or "foo-bar", so we
                 * don't want to do our own scan.
                 */
                strbuf_add(&work_path, name, strlen(name));
                strbuf_addch(&work_path, '/');
                pos = index_name_pos(istate, work_path.buf, work_path.len);
                nr_in_cone = handle_path_with_trailing_slash(
                        istate, work_path.buf, pos);
                strbuf_release(&work_path);
                return nr_in_cone;
        }
}

/*
 * The daemon can decorate directory events, such as a move or rename,
 * by adding a trailing slash to the observed name.  Use this to
 * explicitly invalidate the entire cone under that directory.
 *
 * The daemon can only reliably do that if the OS FSEvent contains
 * sufficient information in the event.
 *
 * macOS FSEvents have enough information.
 *
 * Other platforms may or may not be able to do it (and it might
 * depend on the type of event (for example, a daemon could lstat() an
 * observed pathname after a rename, but not after a delete)).
 *
 * If we find an exact match in the index for a path with a trailing
 * slash, it means that we matched a sparse-index directory in a
 * cone-mode sparse-checkout (since that's the only time we have
 * directories in the index).  We should never see this in practice
 * (because sparse directories should not be present and therefore
 * not generating FS events).  Either way, we can treat them in the
 * same way and just invalidate the cache-entry and the untracked
 * cache (and in this case, the forward cache-entry scan won't find
 * anything and it doesn't hurt to let it run).
 *
 * Return the number of cache-entries that we invalidated.  We will
 * use this later to determine if we need to attempt a second
 * case-insensitive search on case-insensitive file systems.  That is,
 * if the search using the observed-case in the FSEvent yields any
 * results, we assume the prefix is case-correct.  If there are no
 * matches, we still don't know if the observed path is simply
 * untracked or case-incorrect.
 */
static size_t handle_path_with_trailing_slash(
        struct index_state *istate, const char *name, int pos)
{
        int i;
        size_t nr_in_cone = 0;

        /*
         * Mark the untracked cache dirty for this directory path
         * (regardless of whether or not we find an exact match for it
         * in the index or find it to be proper prefix of one or more
         * files in the index), since the FSEvent is hinting that
         * there may be changes on or within the directory.
         */
        untracked_cache_invalidate_trimmed_path(istate, name, 0);

        if (pos < 0)
                pos = -pos - 1;

        /* Mark all entries for the folder invalid */
        for (i = pos; i < istate->cache_nr; i++) {
                if (!starts_with(istate->cache[i]->name, name))
                        break;
                invalidate_ce_fsm(istate->cache[i]);
                nr_in_cone++;
        }

        return nr_in_cone;
}

static void fsmonitor_refresh_callback(struct index_state *istate, char *name)
{
        int len = strlen(name);
        int pos = index_name_pos(istate, name, len);
        size_t nr_in_cone;

        trace_printf_key(&trace_fsmonitor,
                         "fsmonitor_refresh_callback '%s' (pos %d)",
                         name, pos);

        if (name[len - 1] == '/')
                nr_in_cone = handle_path_with_trailing_slash(istate, name, pos);
        else
                nr_in_cone = handle_path_without_trailing_slash(istate, name, pos);

        /*
         * If we did not find an exact match for this pathname or any
         * cache-entries with this directory prefix and we're on a
         * case-insensitive file system, try again using the name-hash
         * and dir-name-hash.
         */
        if (!nr_in_cone && ignore_case) {
                nr_in_cone = handle_using_name_hash_icase(istate, name);
                if (!nr_in_cone)
                        nr_in_cone = handle_using_dir_name_hash_icase(
                                istate, name);
        }

        if (nr_in_cone)
                trace_printf_key(&trace_fsmonitor,
                                 "fsmonitor_refresh_callback CNT: %d",
                                 (int)nr_in_cone);
}

/*
 * The number of pathnames that we need to receive from FSMonitor
 * before we force the index to be updated.
 *
 * Note that any pathname within the set of received paths MAY cause
 * cache-entry or istate flag bits to be updated and thus cause the
 * index to be updated on disk.
 *
 * However, the response may contain many paths (such as ignored
 * paths) that will not update any flag bits.  And thus not force the
 * index to be updated.  (This is fine and normal.)  It also means
 * that the token will not be updated in the FSMonitor index
 * extension.  So the next Git command will find the same token in the
 * index, make the same token-relative request, and receive the same
 * response (plus any newly changed paths).  If this response is large
 * (and continues to grow), performance could be impacted.
 *
 * For example, if the user runs a build and it writes 100K object
 * files but doesn't modify any source files, the index would not need
 * to be updated.  The FSMonitor response (after the build and
 * relative to a pre-build token) might be 5MB.  Each subsequent Git
 * command will receive that same 100K/5MB response until something
 * causes the index to be updated.  And `refresh_fsmonitor()` will
 * have to iterate over those 100K paths each time.
 *
 * Performance could be improved if we optionally force update the
 * index after a very large response and get an updated token into
 * the FSMonitor index extension.  This should allow subsequent
 * commands to get smaller and more current responses.
 *
 * The value chosen here does not need to be precise.  The index
 * will be updated automatically the first time the user touches
 * a tracked file and causes a command like `git status` to
 * update an mtime to be updated and/or set a flag bit.
 */
static int fsmonitor_force_update_threshold = 100;

void refresh_fsmonitor(struct index_state *istate)
{
        static int warn_once = 0;
        struct strbuf query_result = STRBUF_INIT;
        int query_success = 0, hook_version = -1;
        size_t bol = 0; /* beginning of line */
        uint64_t last_update;
        struct strbuf last_update_token = STRBUF_INIT;
        char *buf;
        unsigned int i;
        int is_trivial = 0;
        struct repository *r = istate->repo;
        enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(r);
        enum fsmonitor_reason reason = fsm_settings__get_reason(r);

        if (!warn_once && reason > FSMONITOR_REASON_OK) {
                char *msg = fsm_settings__get_incompatible_msg(r, reason);
                warn_once = 1;
                warning("%s", msg);
                free(msg);
        }

        if (fsm_mode <= FSMONITOR_MODE_DISABLED ||
            istate->fsmonitor_has_run_once)
                return;

        istate->fsmonitor_has_run_once = 1;

        trace_printf_key(&trace_fsmonitor, "refresh fsmonitor");

        if (fsm_mode == FSMONITOR_MODE_IPC) {
                query_success = !fsmonitor_ipc__send_query(
                        istate->fsmonitor_last_update ?
                        istate->fsmonitor_last_update : "builtin:fake",
                        &query_result);
                if (query_success) {
                        /*
                         * The response contains a series of nul terminated
                         * strings.  The first is the new token.
                         *
                         * Use `char *buf` as an interlude to trick the CI
                         * static analysis to let us use `strbuf_addstr()`
                         * here (and only copy the token) rather than
                         * `strbuf_addbuf()`.
                         */
                        buf = query_result.buf;
                        strbuf_addstr(&last_update_token, buf);
                        bol = last_update_token.len + 1;
                        is_trivial = query_result.buf[bol] == '/';
                        if (is_trivial)
                                trace2_data_intmax("fsm_client", NULL,
                                                   "query/trivial-response", 1);
                } else {
                        /*
                         * The builtin daemon is not available on this
                         * platform -OR- we failed to get a response.
                         *
                         * Generate a fake token (rather than a V1
                         * timestamp) for the index extension.  (If
                         * they switch back to the hook API, we don't
                         * want ambiguous state.)
                         */
                        strbuf_addstr(&last_update_token, "builtin:fake");
                }

                goto apply_results;
        }

        assert(fsm_mode == FSMONITOR_MODE_HOOK);

        hook_version = fsmonitor_hook_version();

        /*
         * This could be racy so save the date/time now and query_fsmonitor_hook
         * should be inclusive to ensure we don't miss potential changes.
         */
        last_update = getnanotime();
        if (hook_version == HOOK_INTERFACE_VERSION1)
                strbuf_addf(&last_update_token, "%"PRIu64"", last_update);

        /*
         * If we have a last update token, call query_fsmonitor_hook for the set of
         * changes since that token, else assume everything is possibly dirty
         * and check it all.
         */
        if (istate->fsmonitor_last_update) {
                if (hook_version == -1 || hook_version == HOOK_INTERFACE_VERSION2) {
                        query_success = !query_fsmonitor_hook(
                                r, HOOK_INTERFACE_VERSION2,
                                istate->fsmonitor_last_update, &query_result);

                        if (query_success) {
                                if (hook_version < 0)
                                        hook_version = HOOK_INTERFACE_VERSION2;

                                /*
                                 * First entry will be the last update token
                                 * Need to use a char * variable because static
                                 * analysis was suggesting to use strbuf_addbuf
                                 * but we don't want to copy the entire strbuf
                                 * only the chars up to the first NUL
                                 */
                                buf = query_result.buf;
                                strbuf_addstr(&last_update_token, buf);
                                if (!last_update_token.len) {
                                        warning("Empty last update token.");
                                        query_success = 0;
                                } else {
                                        bol = last_update_token.len + 1;
                                        is_trivial = query_result.buf[bol] == '/';
                                }
                        } else if (hook_version < 0) {
                                hook_version = HOOK_INTERFACE_VERSION1;
                                if (!last_update_token.len)
                                        strbuf_addf(&last_update_token, "%"PRIu64"", last_update);
                        }
                }

                if (hook_version == HOOK_INTERFACE_VERSION1) {
                        query_success = !query_fsmonitor_hook(
                                r, HOOK_INTERFACE_VERSION1,
                                istate->fsmonitor_last_update, &query_result);
                        if (query_success)
                                is_trivial = query_result.buf[0] == '/';
                }

                if (is_trivial)
                        trace2_data_intmax("fsm_hook", NULL,
                                           "query/trivial-response", 1);

                trace_performance_since(last_update, "fsmonitor process '%s'",
                                        fsm_settings__get_hook_path(r));
                trace_printf_key(&trace_fsmonitor,
                                 "fsmonitor process '%s' returned %s",
                                 fsm_settings__get_hook_path(r),
                                 query_success ? "success" : "failure");
        }

apply_results:
        /*
         * The response from FSMonitor (excluding the header token) is
         * either:
         *
         * [a] a (possibly empty) list of NUL delimited relative
         *     pathnames of changed paths.  This list can contain
         *     files and directories.  Directories have a trailing
         *     slash.
         *
         * [b] a single '/' to indicate the provider had no
         *     information and that we should consider everything
         *     invalid.  We call this a trivial response.
         */
        trace2_region_enter("fsmonitor", "apply_results", istate->repo);

        if (query_success && !is_trivial) {
                /*
                 * Mark all pathnames returned by the monitor as dirty.
                 *
                 * This updates both the cache-entries and the untracked-cache.
                 */
                int count = 0;

                buf = query_result.buf;
                for (i = bol; i < query_result.len; i++) {
                        if (buf[i] != '\0')
                                continue;
                        fsmonitor_refresh_callback(istate, buf + bol);
                        bol = i + 1;
                        count++;
                }
                if (bol < query_result.len) {
                        fsmonitor_refresh_callback(istate, buf + bol);
                        count++;
                }

                /* Now mark the untracked cache for fsmonitor usage */
                if (istate->untracked)
                        istate->untracked->use_fsmonitor = 1;

                if (count > fsmonitor_force_update_threshold)
                        istate->cache_changed |= FSMONITOR_CHANGED;

                trace2_data_intmax("fsmonitor", istate->repo, "apply_count",
                                   count);

        } else {
                /*
                 * We failed to get a response or received a trivial response,
                 * so invalidate everything.
                 *
                 * We only want to run the post index changed hook if
                 * we've actually changed entries, so keep track if we
                 * actually changed entries or not.
                 */
                int is_cache_changed = 0;

                for (i = 0; i < istate->cache_nr; i++) {
                        if (istate->cache[i]->ce_flags & CE_FSMONITOR_VALID) {
                                is_cache_changed = 1;
                                istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;
                        }
                }

                /*
                 * If we're going to check every file, ensure we save
                 * the results.
                 */
                if (is_cache_changed)
                        istate->cache_changed |= FSMONITOR_CHANGED;

                if (istate->untracked)
                        istate->untracked->use_fsmonitor = 0;
        }
        trace2_region_leave("fsmonitor", "apply_results", istate->repo);

        strbuf_release(&query_result);

        /* Now that we've updated istate, save the last_update_token */
        FREE_AND_NULL(istate->fsmonitor_last_update);
        istate->fsmonitor_last_update = strbuf_detach(&last_update_token, NULL);
}

/*
 * The caller wants to turn on FSMonitor.  And when the caller writes
 * the index to disk, a FSMonitor extension should be included.  This
 * requires that `istate->fsmonitor_last_update` not be NULL.  But we
 * have not actually talked to a FSMonitor process yet, so we don't
 * have an initial value for this field.
 *
 * For a protocol V1 FSMonitor process, this field is a formatted
 * "nanoseconds since epoch" field.  However, for a protocol V2
 * FSMonitor process, this field is an opaque token.
 *
 * Historically, `add_fsmonitor()` has initialized this field to the
 * current time for protocol V1 processes.  There are lots of race
 * conditions here, but that code has shipped...
 *
 * The only true solution is to use a V2 FSMonitor and get a current
 * or default token value (that it understands), but we cannot do that
 * until we have actually talked to an instance of the FSMonitor process
 * (but the protocol requires that we send a token first...).
 *
 * For simplicity, just initialize like we have a V1 process and require
 * that V2 processes adapt.
 */
static void initialize_fsmonitor_last_update(struct index_state *istate)
{
        struct strbuf last_update = STRBUF_INIT;

        strbuf_addf(&last_update, "%"PRIu64"", getnanotime());
        istate->fsmonitor_last_update = strbuf_detach(&last_update, NULL);
}

void add_fsmonitor(struct index_state *istate)
{
        unsigned int i;

        if (!istate->fsmonitor_last_update) {
                trace_printf_key(&trace_fsmonitor, "add fsmonitor");
                istate->cache_changed |= FSMONITOR_CHANGED;
                initialize_fsmonitor_last_update(istate);

                /* reset the fsmonitor state */
                for (i = 0; i < istate->cache_nr; i++)
                        istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;

                /* reset the untracked cache */
                if (istate->untracked) {
                        add_untracked_cache(istate);
                        istate->untracked->use_fsmonitor = 1;
                }

                /* Update the fsmonitor state */
                refresh_fsmonitor(istate);
        }
}

void remove_fsmonitor(struct index_state *istate)
{
        if (istate->fsmonitor_last_update) {
                trace_printf_key(&trace_fsmonitor, "remove fsmonitor");
                istate->cache_changed |= FSMONITOR_CHANGED;
                FREE_AND_NULL(istate->fsmonitor_last_update);
        }
}

void tweak_fsmonitor(struct index_state *istate)
{
        unsigned int i;
        int fsmonitor_enabled = (fsm_settings__get_mode(istate->repo)
                                 > FSMONITOR_MODE_DISABLED);

        if (istate->fsmonitor_dirty) {
                if (fsmonitor_enabled) {
                        /* Mark all entries valid */
                        for (i = 0; i < istate->cache_nr; i++) {
                                if (S_ISGITLINK(istate->cache[i]->ce_mode))
                                        continue;
                                istate->cache[i]->ce_flags |= CE_FSMONITOR_VALID;
                        }

                        /* Mark all previously saved entries as dirty */
                        assert_index_minimum(istate, istate->fsmonitor_dirty->bit_size);
                        ewah_each_bit(istate->fsmonitor_dirty, fsmonitor_ewah_callback, istate);

                        refresh_fsmonitor(istate);
                }

                ewah_free(istate->fsmonitor_dirty);
                istate->fsmonitor_dirty = NULL;
        }

        if (fsmonitor_enabled)
                add_fsmonitor(istate);
        else
                remove_fsmonitor(istate);
}