From 5fee84b972a381301cd78f42e1190115c4bed8a5 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Wed, 5 Feb 2025 16:52:15 -0500
Subject: stat: Expose mount IDs from statx()

---
 src/ioq.c  |  2 +-
 src/stat.c | 31 ++++++++++++++++++++++++++++++-
 src/stat.h |  9 +++++++++
 3 files changed, 40 insertions(+), 2 deletions(-)

(limited to 'src')

diff --git a/src/ioq.c b/src/ioq.c
index 6fc4cbb..0e7ac42 100644
--- a/src/ioq.c
+++ b/src/ioq.c
@@ -846,7 +846,7 @@ static struct io_uring_sqe *ioq_dispatch_async(struct ioq_ring_state *state, str
 			sqe = ioq_get_sqe(state);
 			struct ioq_stat *args = &ent->stat;
 			int flags = bfs_statx_flags(args->flags);
-			unsigned int mask = STATX_BASIC_STATS | STATX_BTIME;
+			unsigned int mask = bfs_statx_mask();
 			io_uring_prep_statx(sqe, args->dfd, args->path, flags, mask, args->xbuf);
 		}
 #endif
diff --git a/src/stat.c b/src/stat.c
index e99e711..1fcfde3 100644
--- a/src/stat.c
+++ b/src/stat.c
@@ -51,6 +51,8 @@ const char *bfs_stat_field_name(enum bfs_stat_field field) {
 		return "change time";
 	case BFS_STAT_MTIME:
 		return "modification time";
+	case BFS_STAT_MNT_ID:
+		return "mount ID";
 	}
 
 	bfs_bug("Unrecognized stat field %d", (int)field);
@@ -101,6 +103,10 @@ void bfs_stat_convert(struct bfs_stat *dest, const struct stat *src) {
 	dest->rdev = src->st_rdev;
 	dest->mask |= BFS_STAT_RDEV;
 
+	// No mount IDs in regular stat(), so use the dev_t as an approximation
+	dest->mnt_id = dest->dev;
+	dest->mask |= BFS_STAT_MNT_ID;
+
 #if BFS_HAS_ST_FLAGS
 	dest->attrs = src->st_flags;
 	dest->mask |= BFS_STAT_ATTRS;
@@ -169,6 +175,17 @@ int bfs_statx_flags(enum bfs_stat_flags flags) {
 	return ret;
 }
 
+unsigned int bfs_statx_mask(void) {
+	unsigned int mask = STATX_BASIC_STATS | STATX_BTIME;
+#ifdef STATX_MNT_ID
+	mask |= STATX_MNT_ID;
+#endif
+#ifdef STATX_MNT_ID_UNIQUE
+	mask |= STATX_MNT_ID_UNIQUE;
+#endif
+	return mask;
+}
+
 int bfs_statx_convert(struct bfs_stat *dest, const struct statx *src) {
 	// Callers shouldn't have to check anything except the times
 	const unsigned int guaranteed = STATX_BASIC_STATS & ~(STATX_ATIME | STATX_CTIME | STATX_MTIME);
@@ -209,6 +226,18 @@ int bfs_statx_convert(struct bfs_stat *dest, const struct statx *src) {
 	dest->attrs = src->stx_attributes;
 	dest->mask |= BFS_STAT_ATTRS;
 
+	dest->mnt_id = dest->dev;
+#ifdef STATX_MNT_ID
+	unsigned int mnt_mask = STATX_MNT_ID;
+#  ifdef STATX_MNT_ID_UNIQUE
+	mnt_mask |= STATX_MNT_ID_UNIQUE;
+#  endif
+	if (src->stx_mask & mnt_mask) {
+		dest->mnt_id = src->stx_mnt_id;
+	}
+#endif
+	dest->mask |= BFS_STAT_MNT_ID;
+
 	if (src->stx_mask & STATX_ATIME) {
 		dest->atime.tv_sec = src->stx_atime.tv_sec;
 		dest->atime.tv_nsec = src->stx_atime.tv_nsec;
@@ -240,7 +269,7 @@ int bfs_statx_convert(struct bfs_stat *dest, const struct statx *src) {
  * bfs_stat() implementation backed by statx().
  */
 static int bfs_statx_impl(int at_fd, const char *at_path, int at_flags, struct bfs_stat *buf) {
-	unsigned int mask = STATX_BASIC_STATS | STATX_BTIME;
+	unsigned int mask = bfs_statx_mask();
 	struct statx xbuf;
 	int ret = bfs_statx(at_fd, at_path, at_flags, mask, &xbuf);
 	if (ret != 0) {
diff --git a/src/stat.h b/src/stat.h
index 50c91de..c4a63d3 100644
--- a/src/stat.h
+++ b/src/stat.h
@@ -14,6 +14,7 @@
 
 #include "bfs.h"
 
+#include <stdint.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <time.h>
@@ -56,6 +57,7 @@ enum bfs_stat_field {
 	BFS_STAT_BTIME  = 1 << 11,
 	BFS_STAT_CTIME  = 1 << 12,
 	BFS_STAT_MTIME  = 1 << 13,
+	BFS_STAT_MNT_ID = 1 << 14,
 };
 
 /**
@@ -102,6 +104,8 @@ struct bfs_stat {
 	blkcnt_t blocks;
 	/** The device ID represented by this file. */
 	dev_t rdev;
+	/** The ID of the mount point containing this file. */
+	uint64_t mnt_id;
 
 	/** Attributes/flags set on the file. */
 	unsigned long long attrs;
@@ -149,6 +153,11 @@ void bfs_stat_convert(struct bfs_stat *dest, const struct stat *src);
  */
 int bfs_statx_flags(enum bfs_stat_flags flags);
 
+/**
+ * Get the default statx() mask.
+ */
+unsigned int bfs_statx_mask(void);
+
 /**
  * Convert struct statx to struct bfs_stat.
  */
-- 
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