For announced JPEGs: write to a temporary file first and then (atomically)

rename it to the real output file. This fixes problems with a threaded HTTPD
where JPEG files could be downloaded while they were still being written to.


git-svn-id: http://svn.cactuscode.org/arrangements/CactusIO/IOJpeg/trunk@116 eff87b29-5268-4891-90a3-a07138403961

1 files changed, 40 insertions, 18 deletions

diff --git a/src/Write.c b/src/Write.c
index 4005fd9..faf87ef 100644
--- a/src/Write.c
+++ b/src/Write.c
@@ -313,7 +313,7 @@ static void WriteData (const cGH *GH, int vindex, const char *alias, int dim,
   unsigned char *dataout;
   const ioJpegGH *myGH;
   ioAdvertisedFileDesc advertised_file;
-  char *filename, *fullname;
+  char *filename, *tmpfilename, *fullname;
   char slicename[30];
   const char *extensions[] = {"xy", "xz", "yz"};
   DECLARE_CCTK_PARAMETERS
@@ -338,50 +338,72 @@ static void WriteData (const cGH *GH, int vindex, const char *alias, int dim,
     sprintf (slicename, "%s_[%d]", extensions[dir], myGH->sp2xyz[dim-1][dir]);
   }
 
-  filename = (char *) malloc (strlen (myGH->out_dir) + strlen (alias) +
-                              sizeof (slicename) + 20);
+  filename = malloc (strlen (myGH->out_dir) + strlen (alias) +
+                     sizeof (slicename) + 20);
 
   if (CCTK_Equals (mode, "remove"))
   {
     sprintf (filename, "%s%s_%s.jpeg", myGH->out_dir, alias, slicename);
+    tmpfilename = malloc (strlen (filename) + 5);
+    sprintf (tmpfilename, "%s.tmp", filename);
   }
   else
   {
     sprintf (filename, "%s%s_%s.it_%d.jpeg", myGH->out_dir, alias, slicename,
              GH->cctk_iteration);
+    tmpfilename = NULL;
   }
 
-  file = fopen (filename, "w");
+  /* Write a JPEG file to be advertised to a temporary file first
+     and rename it later.
+     This fixes a racing problem with HTTPD when running with pthreads support:
+     a file could be downloaded (through the HTTPD thread) while is was still
+     being written to (in the main simulation thread).
+     Now the JPEG is written to a temporary file first and then (atomically)
+     renamed. */
+  file = fopen (tmpfilename ? tmpfilename : filename, "w");
   if (file)
   {
     /* write the data */
     WriteJPEGToFileRGB (hsize[0], hsize[1], dataout, colormap_quality, file);
 
-    /* advertise the file for downloading */
-    if (CCTK_Equals (mode, "remove") && myGH->out_last[vindex] < 0)
+    /* close the file */
+    fclose (file);
+
+    /* in "remove" mode: rename and advertise the file for downloading */
+    if (tmpfilename)
     {
-      fullname = CCTK_FullName (vindex);
-      advertised_file.slice = slicename;
-      advertised_file.thorn = CCTK_THORNSTRING;
-      advertised_file.varname = fullname;
-      advertised_file.description = "Jpegs of slices";
-      advertised_file.mimetype = "image/jpeg";
+      if (rename (tmpfilename, filename))
+      {
+        CCTK_VWarn (1, __LINE__, __FILE__, CCTK_THORNSTRING,
+                    "Cannot rename temporary output file '%s' into '%s'",
+                    tmpfilename, filename);
+      }
+      else if (myGH->out_last[vindex] < 0)
+      {
+        fullname = CCTK_FullName (vindex);
+        advertised_file.slice = slicename;
+        advertised_file.thorn = CCTK_THORNSTRING;
+        advertised_file.varname = fullname;
+        advertised_file.description = "Jpegs of slices";
+        advertised_file.mimetype = "image/jpeg";
 
-      IOUtil_AdvertiseFile (GH, filename, &advertised_file);
+        IOUtil_AdvertiseFile (GH, filename, &advertised_file);
 
-      free (fullname);
+        free (fullname);
+      }
     }
-
-    /* close the file */
-    fclose (file);
   }
   else
   {
     CCTK_VWarn (1, __LINE__, __FILE__, CCTK_THORNSTRING,
-                "Cannot open IOJpeg output file '%s'", filename);
+                "Cannot open IOJpeg %s output file '%s'",
+                tmpfilename ? "temporary" : "",
+                tmpfilename ? tmpfilename : filename);
   }
 
   /* clean up */
   free (dataout);
+  free (tmpfilename);
   free (filename);
 }