Sched: make get_file_size request in file_upload_handler read-only

* If there is a size discrepancy between server and client this needs to be handled in handle_file_upload()
* Setting a write lock on a read only file does not work obviously. Instead try to see if another process has a write lock and continue if not. If another process is still writing to the file there is no need to determine the size.

sched/file_upload_handler.cpp

@@ -475,7 +475,7 @@ int handle_get_file_size(char* file_name) {
         return return_error(ERR_TRANSIENT, "Server is out of disk space");
     }
 
-    fd = open(path, O_WRONLY|O_APPEND);
+    fd = open(path, O_RDONLY);
 
     if (fd<0 && ENOENT==errno) {
         // file does not exist: return zero length
@@ -495,8 +495,8 @@ int handle_get_file_size(char* file_name) {
         );
         return return_error(ERR_TRANSIENT, "can't open file");
     }
-
-    if ((pid = mylockf(fd))) {
+#ifdef LOCK_FILES
+    if ((pid = checklockf(fd))) {
         // file locked by another file_upload_handler: try again later
         //
         close(fd);
@@ -507,7 +507,8 @@ int handle_get_file_size(char* file_name) {
             "[%s] locked by file_upload_handler PID=%d", file_name, pid
         );
     }
-    // file exists, writable, not locked by anyone else, so return length.
+#endif
+    // file exists, readable, not locked by anyone else, so return length.
     //
     retval = stat(path, &sbuf);
     close(fd);

sched/sched_util_basic.cpp

@@ -239,6 +239,24 @@ int mylockf(int fd) {
     return -1;
 }
 
+// check if there is a write lock on the given file with given fd.  Returns:
+// 0 if there is no write lock
+// PID (>0) of the process that has the lock
+// -1 if error
+//
+int checklockf(int fd) {
+    struct flock fl;
+    fl.l_type=F_RDLCK;
+    fl.l_whence=SEEK_SET;
+    fl.l_start=0;
+    fl.l_len=0;
+    if (-1 != fcntl(fd, F_GETLK, &fl)) {
+        if (fl.l_type == F_UNLCK) return 0;
+        if (fl.l_pid>0) return fl.l_pid;
+    }
+    return -1;
+}
+
 bool is_arg(const char* x, const char* y) {
     char buf[256];
     strcpy(buf, "--");

sched/sched_util_basic.h

@@ -60,6 +60,12 @@ extern int dir_hier_url(
 //
 extern int mylockf(int fd);
 
+// returns zero if there is no write lock on file with file descriptor fd.
+// returns < 0 if error
+// returns PID > 0 of the process that has the lock
+//
+extern int checklockf(int fd);
+
 // return true if x is -y or --y (for argv processing)
 //
 extern bool is_arg(const char*, const char*);