Return non-zero exit status if "compare" finds differences

This makes `metastore` more useful in scripts, just to check if it found
anything, in the same way as `diff` and `grep`.

Note that POSIX requires that EXIT_SUCCESS is 0, and EXIT_FAILURE is 1,
so I've defined EXIT_DIFFERENCES as 2 which, while different from the
values `diff` and `grep` use (1 for finding something, 2 for errors),
maintains a certain amount of backwards compatibility with previous
versions of metastore.

4 files changed, 21 insertions, 7 deletions

diff --git a/src/metaentry.c b/src/metaentry.c
index 7e90290..37069b0 100644
--- a/src/metaentry.c
+++ b/src/metaentry.c
@@ -875,7 +875,7 @@ mentry_compare(struct metaentry *left, struct metaentry *right, msettings *st)
 }
 
 /* Compares lists of real and stored metadata and calls pfunc for each */
-void
+int
 mentries_compare(struct metahash *mhashreal,
                  struct metahash *mhashstored,
                  void (*pfunc)
@@ -884,10 +884,11 @@ mentries_compare(struct metahash *mhashreal,
 {
 	struct metaentry *real, *stored;
 	int key;
+	int cmp, diffs = 0;
 
 	if (!mhashreal || !mhashstored) {
 		msg(MSG_ERROR, "%s called with empty list\n", __func__);
-		return;
+		return -1;
 	}
 
 	for (key = 0; key < HASH_INDEXES; key++) {
@@ -895,18 +896,27 @@ mentries_compare(struct metahash *mhashreal,
 			stored = mentry_find(real->path, mhashstored);
 
 			if (!stored)
-				pfunc(real, NULL, DIFF_ADDED);
+				cmp = DIFF_ADDED;
 			else
-				pfunc(real, stored, mentry_compare(real, stored, st));
+				cmp = mentry_compare(real, stored, st);
+
+			if (cmp > 0)
+				++diffs;
+
+			pfunc(real, stored, cmp);
 		}
 
 		for (stored = mhashstored->bucket[key]; stored; stored = stored->next) {
 			real = mentry_find(stored->path, mhashreal);
 
-			if (!real)
+			if (!real) {
+				++diffs;
 				pfunc(NULL, stored, DIFF_DELE);
+			}
 		}
 	}
+
+	return diffs;
 }
 
 /* Dumps given metadata */
diff --git a/src/metaentry.h b/src/metaentry.h
index fd28c17..497cbc2 100644
--- a/src/metaentry.h
+++ b/src/metaentry.h
@@ -87,7 +87,7 @@ int mentry_compare(struct metaentry *left,
                    msettings *st);
 
 /* Compares lists of real and stored metadata and calls pfunc for each */
-void mentries_compare(struct metahash *mhashreal,
+int mentries_compare(struct metahash *mhashreal,
                       struct metahash *mhashstored,
                       void (*pfunc)(struct metaentry *real,
                                     struct metaentry *stored,
diff --git a/src/metastore.c b/src/metastore.c
index 2f6d9bc..fb70834 100644
--- a/src/metastore.c
+++ b/src/metastore.c
@@ -572,7 +572,8 @@ main(int argc, char **argv)
 
 	switch (action) {
 	case ACTION_DIFF:
-		mentries_compare(real, stored, compare_print, &settings);
+		if (mentries_compare(real, stored, compare_print, &settings) > 0)
+			exit(EXIT_DIFFERENCES);
 		break;
 	case ACTION_SAVE:
 		mentries_tofile(real, settings.metafile, settings.format);
diff --git a/src/metastore.h b/src/metastore.h
index de51292..b7d588e 100644
--- a/src/metastore.h
+++ b/src/metastore.h
@@ -46,6 +46,9 @@
 #define ACTIONS_READING 0x07
 #define ACTIONS_WRITING 0x70
 
+/* Exit statuses (statii?) other than SUCCESS (0) and FAILURE (1) */
+#define EXIT_DIFFERENCES	2
+
 /* Possible build defines */
 #ifndef   NO_XATTR
 # define  NO_XATTR 0