avformat/utils: fix ff_mkdir_p() swallowing intermediate mkdir errors

When creating nested directories (e.g. /a/b/c), a genuine mkdir()
failure for an intermediate component was overwritten by later
attempts, making the original failure harder to diagnose.

Stop immediately on intermediate errors other than EEXIST, preserving
errno for the caller. Existing path components remain non-fatal, as
required by mkdir -p semantics. Add a regression test for creating a
child below an existing parent directory.

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavformat/Makefile

@@ -780,6 +780,7 @@ SKIPHEADERS-$(CONFIG_FFRTMPCRYPT_PROTOCOL) += rtmpdh.h
 SKIPHEADERS-$(CONFIG_NETWORK)            += network.h rtsp.h
 
 TESTPROGS = id3v2                                                       \
+            mkdir                                                       \
             seek                                                        \
             url                                                         \
             seek_utils

libavformat/tests/.gitignore

@@ -1,6 +1,7 @@
 /id3v2
 /fifo_muxer
 /imf
+/mkdir
 /movenc
 /noproxy
 /rtmpdh

libavformat/tests/mkdir.c

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2026 Jun Zhao
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "config.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+#if HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
+#include "libavutil/random_seed.h"
+
+#include "libavformat/internal.h"
+#include "libavformat/os_support.h"
+
+int main(void)
+{
+    char parent[64];
+    char child[80];
+
+    snprintf(parent, sizeof(parent), "ff-mkdir-test-%08x", av_get_random_seed());
+    snprintf(child, sizeof(child), "%s/child", parent);
+
+    if (mkdir(parent, 0755) < 0) {
+        perror("mkdir parent");
+        return 1;
+    }
+
+    if (ff_mkdir_p(child) < 0) {
+        perror("ff_mkdir_p");
+        rmdir(parent);
+        return 1;
+    }
+
+    if (rmdir(child) < 0) {
+        perror("rmdir child");
+        rmdir(parent);
+        return 1;
+    }
+
+    if (rmdir(parent) < 0) {
+        perror("rmdir parent");
+        return 1;
+    }
+
+    return 0;
+}

libavformat/utils.c

@@ -443,6 +443,12 @@ int ff_mkdir_p(const char *path)
             tmp_ch = *pos;
             *pos = '\0';
             ret = mkdir(temp, 0755);
+            if (ret < 0 && errno != EEXIST) {
+                int err = errno;
+                av_free(temp);
+                errno = err;
+                return ret;
+            }
             *pos = tmp_ch;
         }
     }

tests/fate/libavformat.mak

@@ -2,6 +2,11 @@
 #fate-async: libavformat/tests/async$(EXESUF)
 #fate-async: CMD = run libavformat/tests/async
 
+FATE_LIBAVFORMAT += fate-mkdir
+fate-mkdir: libavformat/tests/mkdir$(EXESUF)
+fate-mkdir: CMD = run libavformat/tests/mkdir$(EXESUF)
+fate-mkdir: CMP = null
+
 FATE_LIBAVFORMAT-$(CONFIG_NETWORK) += fate-noproxy
 fate-noproxy: libavformat/tests/noproxy$(EXESUF)
 fate-noproxy: CMD = run libavformat/tests/noproxy$(EXESUF)