index: make index.threads=true enable ieot and eoie

If a user explicitly sets

	[index]
		threads = true

to read the index using multiple threads, ensure that index writes
include the offset table by default to make that possible.  This
ensures that the user's intent of turning on threading is respected.

In other words, permit the following configurations:

- index.threads and index.recordOffsetTable unspecified: do not write
  the offset table yet (to avoid alarming the user with "ignoring IEOT
  extension" messages when an older version of Git accesses the
  repository) but do make use of multiple threads to read the index if
  the supporting offset table is present.

  This can also be requested explicitly by setting index.threads=true,
  0, or >1 and index.recordOffsetTable=false.

- index.threads=false or 1: do not write the offset table, and do not
  make use of the offset table.

  One can set index.recordOffsetTable=false as well, to be more
  explicit.

- index.threads=true, 0, or >1 and index.recordOffsetTable unspecified:
  write the offset table and make use of threads at read time.

  This can also be requested by setting index.threads=true, 0, >1, or
  unspecified and index.recordOffsetTable=true.

Fortunately the complication is temporary: once most Git installations
have upgraded to a version with support for the IEOT and EOIE
extensions, we can flip the defaults for index.recordEndOfIndexEntries
and index.recordOffsetTable to true and eliminate the settings.

Helped-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/index.txt

@@ -3,14 +3,16 @@ index.recordEndOfIndexEntries::
 	Entry" section. This reduces index load time on multiprocessor
 	machines but produces a message "ignoring EOIE extension" when
 	reading the index using Git versions before 2.20. Defaults to
-	'false'.
+	'true' if index.threads has been explicitly enabled, 'false'
+	otherwise.
 
 index.recordOffsetTable::
 	Specifies whether the index file should include an "Index Entry
 	Offset Table" section. This reduces index load time on
 	multiprocessor machines but produces a message "ignoring IEOT
 	extension" when reading the index using Git versions before 2.20.
-	Defaults to 'false'.
+	Defaults to 'true' if index.threads has been explicitly enabled,
+	'false' otherwise.
 
 index.threads::
 	Specifies the number of threads to spawn when loading the index.

config.c

@@ -2294,22 +2294,25 @@ int git_config_get_fsmonitor(void)
 	return 0;
 }
 
-int git_config_get_index_threads(void)
+int git_config_get_index_threads(int *dest)
 {
-	int is_bool, val = 0;
+	int is_bool, val;
 
 	val = git_env_ulong("GIT_TEST_INDEX_THREADS", 0);
-	if (val)
-		return val;
+	if (val) {
+		*dest = val;
+		return 0;
+	}
 
 	if (!git_config_get_bool_or_int("index.threads", &is_bool, &val)) {
 		if (is_bool)
-			return val ? 0 : 1;
+			*dest = val ? 0 : 1;
 		else
-			return val;
+			*dest = val;
+		return 0;
 	}
 
-	return 0; /* auto */
+	return 1;
 }
 
 NORETURN

config.h

@@ -246,11 +246,11 @@ extern int git_config_get_bool(const char *key, int *dest);
 extern int git_config_get_bool_or_int(const char *key, int *is_bool, int *dest);
 extern int git_config_get_maybe_bool(const char *key, int *dest);
 extern int git_config_get_pathname(const char *key, const char **dest);
+extern int git_config_get_index_threads(int *dest);
 extern int git_config_get_untracked_cache(void);
 extern int git_config_get_split_index(void);
 extern int git_config_get_max_percent_split_change(void);
 extern int git_config_get_fsmonitor(void);
-extern int git_config_get_index_threads(void);
 
 /* This dies if the configured or default date is in the future */
 extern int git_config_get_expiry(const char *key, const char **output);

read-cache.c

@@ -2176,7 +2176,8 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 
 	src_offset = sizeof(*hdr);
 
-	nr_threads = git_config_get_index_threads();
+	if (git_config_get_index_threads(&nr_threads))
+		nr_threads = 1;
 
 	/* TODO: does creating more threads than cores help? */
 	if (!nr_threads) {
@@ -2695,7 +2696,13 @@ static int record_eoie(void)
 
 	if (!git_config_get_bool("index.recordendofindexentries", &val))
 		return val;
-	return 0;
+
+	/*
+	 * As a convenience, the end of index entries extension
+	 * used for threading is written by default if the user
+	 * explicitly requested threaded index reads.
+	 */
+	return !git_config_get_index_threads(&val) && val != 1;
 }
 
 static int record_ieot(void)
@@ -2704,7 +2711,13 @@ static int record_ieot(void)
 
 	if (!git_config_get_bool("index.recordoffsettable", &val))
 		return val;
-	return 0;
+
+	/*
+	 * As a convenience, the offset table used for threading is
+	 * written by default if the user explicitly requested
+	 * threaded index reads.
+	 */
+	return !git_config_get_index_threads(&val) && val != 1;
 }
 
 /*
@@ -2765,9 +2778,7 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 	if (ce_write(&c, newfd, &hdr, sizeof(hdr)) < 0)
 		return -1;
 
-	if (HAVE_THREADS)
-		nr_threads = git_config_get_index_threads();
-	else
+	if (!HAVE_THREADS || git_config_get_index_threads(&nr_threads))
 		nr_threads = 1;
 
 	if (nr_threads != 1 && record_ieot()) {