block: report errno when flock fcntl fails

When a call to fcntl(2) for the purpose of adding file locks fails
with an error other than EAGAIN or EACCES, report the error returned
by fcntl.

EAGAIN or EACCES are elided as they are considered to be common
failures, indicating that a conflicting lock is held by another
process.

No errors are elided when removing file locks.

Signed-off-by: David Edmondson <david.edmondson@oracle.com>
Message-Id: <20210113164447.2545785-1-david.edmondson@oracle.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/file-posix.c

@@ -216,6 +216,20 @@ typedef struct RawPosixAIOData {
 static int cdrom_reopen(BlockDriverState *bs);
 #endif
 
+/*
+ * Elide EAGAIN and EACCES details when failing to lock, as this
+ * indicates that the specified file region is already locked by
+ * another process, which is considered a common scenario.
+ */
+#define raw_lock_error_setg_errno(errp, err, fmt, ...)                  \
+    do {                                                                \
+        if ((err) == EAGAIN || (err) == EACCES) {                       \
+            error_setg((errp), (fmt), ## __VA_ARGS__);                  \
+        } else {                                                        \
+            error_setg_errno((errp), (err), (fmt), ## __VA_ARGS__);     \
+        }                                                               \
+    } while (0)
+
 #if defined(__NetBSD__)
 static int raw_normalize_devicepath(const char **filename, Error **errp)
 {
@@ -836,15 +850,16 @@ static int raw_apply_lock_bytes(BDRVRawState *s, int fd,
         if ((perm_lock_bits & bit) && !(locked_perm & bit)) {
             ret = qemu_lock_fd(fd, off, 1, false);
             if (ret) {
-                error_setg(errp, "Failed to lock byte %d", off);
+                raw_lock_error_setg_errno(errp, -ret, "Failed to lock byte %d",
+                                          off);
                 return ret;
             } else if (s) {
                 s->locked_perm |= bit;
             }
         } else if (unlock && (locked_perm & bit) && !(perm_lock_bits & bit)) {
             ret = qemu_unlock_fd(fd, off, 1);
             if (ret) {
-                error_setg(errp, "Failed to unlock byte %d", off);
+                error_setg_errno(errp, -ret, "Failed to unlock byte %d", off);
                 return ret;
             } else if (s) {
                 s->locked_perm &= ~bit;
@@ -857,7 +872,8 @@ static int raw_apply_lock_bytes(BDRVRawState *s, int fd,
         if ((shared_perm_lock_bits & bit) && !(locked_shared_perm & bit)) {
             ret = qemu_lock_fd(fd, off, 1, false);
             if (ret) {
-                error_setg(errp, "Failed to lock byte %d", off);
+                raw_lock_error_setg_errno(errp, -ret, "Failed to lock byte %d",
+                                          off);
                 return ret;
             } else if (s) {
                 s->locked_shared_perm |= bit;
@@ -866,7 +882,7 @@ static int raw_apply_lock_bytes(BDRVRawState *s, int fd,
                    !(shared_perm_lock_bits & bit)) {
             ret = qemu_unlock_fd(fd, off, 1);
             if (ret) {
-                error_setg(errp, "Failed to unlock byte %d", off);
+                error_setg_errno(errp, -ret, "Failed to unlock byte %d", off);
                 return ret;
             } else if (s) {
                 s->locked_shared_perm &= ~bit;
@@ -890,9 +906,10 @@ static int raw_check_lock_bytes(int fd, uint64_t perm, uint64_t shared_perm,
             ret = qemu_lock_fd_test(fd, off, 1, true);
             if (ret) {
                 char *perm_name = bdrv_perm_names(p);
-                error_setg(errp,
-                           "Failed to get \"%s\" lock",
-                           perm_name);
+
+                raw_lock_error_setg_errno(errp, -ret,
+                                          "Failed to get \"%s\" lock",
+                                          perm_name);
                 g_free(perm_name);
                 return ret;
             }
@@ -905,9 +922,10 @@ static int raw_check_lock_bytes(int fd, uint64_t perm, uint64_t shared_perm,
             ret = qemu_lock_fd_test(fd, off, 1, true);
             if (ret) {
                 char *perm_name = bdrv_perm_names(p);
-                error_setg(errp,
-                           "Failed to get shared \"%s\" lock",
-                           perm_name);
+
+                raw_lock_error_setg_errno(errp, -ret,
+                                          "Failed to get shared \"%s\" lock",
+                                          perm_name);
                 g_free(perm_name);
                 return ret;
             }