Enable mkostemp to be weak-linked

mkostemp is not available on some older versions of macOS. In order
for our built binaries to run on them, mkostemp must be weak-linked.
On other systems, we use the autoconf check.

Introduce a function fish_mkstemp_cloexec which uses mkostemp if
it was detected and is available at runtime, else falls back to
mkstemp. This isolates some logic that is currently duplicated in
two places.

See #3138 for more on weak linking.

src/env_universal_common.cpp

@@ -535,15 +535,7 @@ bool env_universal_t::open_temporary_file(const wcstring &directory, wcstring *o
 
     for (size_t attempt = 0; attempt < 10 && !success; attempt++) {
         char *narrow_str = wcs2str(tmp_name_template.c_str());
-#if HAVE_MKOSTEMP
-        int result_fd = mkostemp(narrow_str, O_CLOEXEC);
-#else
-        int result_fd = mkstemp(narrow_str);
-        if (result_fd != -1) {
-            fcntl(result_fd, F_SETFD, FD_CLOEXEC);
-        }
-#endif
-
+        int result_fd = fish_mkstemp_cloexec(narrow_str);
         saved_errno = errno;
         success = result_fd != -1;
         *out_fd = result_fd;

src/fallback.cpp

@@ -91,6 +91,20 @@ char *tparm_solaris_kludge(char *str, ...) {
 
 #endif
 
+int fish_mkstemp_cloexec(char *name_template) {
+#if HAVE_MKOSTEMP
+    // null check because mkostemp may be a weak symbol
+    if (&mkostemp != nullptr) {
+        return mkostemp(name_template, O_CLOEXEC);
+    }
+#endif
+    int result_fd = mkstemp(name_template);
+    if (result_fd != -1) {
+        fcntl(result_fd, F_SETFD, FD_CLOEXEC);
+    }
+    return result_fd;
+}
+
 /// Fallback implementations of wcsdup and wcscasecmp. On systems where these are not needed (e.g.
 /// building on Linux) these should end up just being stripped, as they are static functions that
 /// are not referenced in this file.

src/fallback.h

@@ -23,6 +23,11 @@
 int fish_wcwidth(wchar_t wc);
 int fish_wcswidth(const wchar_t *str, size_t n);
 
+// Replacement for mkostemp(str, O_CLOEXEC)
+// This uses mkostemp if available,
+// otherwise it uses mkstemp followed by fcntl
+int fish_mkstemp_cloexec(char *);
+
 #ifndef WCHAR_MAX
 /// This _should_ be defined by wchar.h, but e.g. OpenBSD doesn't.
 #define WCHAR_MAX INT_MAX

src/history.cpp

@@ -1230,24 +1230,16 @@ bool history_t::save_internal_via_rewrite() {
 
         signal_block();
 
-        // Try to create a temporary file, up to 10 times. We don't use mkstemps because we want to
-        // open it CLO_EXEC. This should almost always succeed on the first try.
+        // Try to create a CLO_EXEC temporary file, up to 10 times.
+        // This should almost always succeed on the first try.
         int out_fd = -1;
         wcstring tmp_name;
         for (size_t attempt = 0; attempt < 10 && out_fd == -1; attempt++) {
             char *narrow_str = wcs2str(tmp_name_template.c_str());
-#if HAVE_MKOSTEMP
-            out_fd = mkostemp(narrow_str, O_CLOEXEC);
+            out_fd = fish_mkstemp_cloexec(narrow_str);
             if (out_fd >= 0) {
                 tmp_name = str2wcstring(narrow_str);
             }
-#else
-            if (narrow_str && mktemp(narrow_str)) {
-                // It was successfully templated; try opening it atomically.
-                tmp_name = str2wcstring(narrow_str);
-                out_fd = wopen_cloexec(tmp_name, O_WRONLY | O_CREAT | O_EXCL | O_TRUNC, 0600);
-            }
-#endif
             free(narrow_str);
         }