altsvc: make saving the cache an atomic operation

... by writing the file to temp name then rename to the final when done.

Assisted-by: Jay Satiro
Fixes #4936
Closes #4942

1 files changed, 33 insertions, 12 deletions

diff --git a/lib/altsvc.c b/lib/altsvc.c
index c05562985..8066302ae 100644
--- a/lib/altsvc.c
+++ b/lib/altsvc.c
@@ -34,6 +34,8 @@
 #include "parsedate.h"
 #include "sendf.h"
 #include "warnless.h"
+#include "rand.h"
+#include "rename.h"
 
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -317,12 +319,15 @@ void Curl_altsvc_cleanup(struct altsvcinfo *altsvc)
 /*
  * Curl_altsvc_save() writes the altsvc cache to a file.
  */
-CURLcode Curl_altsvc_save(struct altsvcinfo *altsvc, const char *file)
+CURLcode Curl_altsvc_save(struct Curl_easy *data,
+                          struct altsvcinfo *altsvc, const char *file)
 {
   struct curl_llist_element *e;
   struct curl_llist_element *n;
   CURLcode result = CURLE_OK;
   FILE *out;
+  char *tempstore;
+  unsigned char randsuffix[9];
 
   if(!altsvc)
     /* no cache activated */
@@ -335,20 +340,36 @@ CURLcode Curl_altsvc_save(struct altsvcinfo *altsvc, const char *file)
   if((altsvc->flags & CURLALTSVC_READONLYFILE) || !file || !file[0])
     /* marked as read-only, no file or zero length file name */
     return CURLE_OK;
-  out = fopen(file, FOPEN_WRITETEXT);
+
+  if(Curl_rand_hex(data, randsuffix, sizeof(randsuffix)))
+    return CURLE_FAILED_INIT;
+
+  tempstore = aprintf("%s.%s.tmp", file, randsuffix);
+  if(!tempstore)
+    return CURLE_OUT_OF_MEMORY;
+
+  out = fopen(tempstore, FOPEN_WRITETEXT);
   if(!out)
-    return CURLE_WRITE_ERROR;
-  fputs("# Your alt-svc cache. https://curl.haxx.se/docs/alt-svc.html\n"
-        "# This file was generated by libcurl! Edit at your own risk.\n",
-        out);
-  for(e = altsvc->list.head; e; e = n) {
-    struct altsvc *as = e->ptr;
-    n = e->next;
-    result = altsvc_out(as, out);
+    result = CURLE_WRITE_ERROR;
+  else {
+    fputs("# Your alt-svc cache. https://curl.haxx.se/docs/alt-svc.html\n"
+          "# This file was generated by libcurl! Edit at your own risk.\n",
+          out);
+    for(e = altsvc->list.head; e; e = n) {
+      struct altsvc *as = e->ptr;
+      n = e->next;
+      result = altsvc_out(as, out);
+      if(result)
+        break;
+    }
+    fclose(out);
+    if(!result && Curl_rename(tempstore, file))
+      result = CURLE_WRITE_ERROR;
+
     if(result)
-      break;
+      unlink(tempstore);
   }
-  fclose(out);
+  free(tempstore);
   return result;
 }