handle stdout correctly if it's a file

When you invoke "tar cfv <tarball> <files>", then tar will close all active file descriptors it has, forks, then opens the target tarball as fd 1 (stdout). Then, it'll exec "gzip -", which *inherits* a stdout which is a file. Now we duplicate this stdout if it's a regular file so that we can advise it later. (The original will be closed via the _exit() routine, probably, which uses direct syscall that we cannot intercept.)

handle stdout correctly if it's a file
fd9969ec · Julius Plenz · e033127e · fd9969ec · fd9969ec
Commit fd9969ec authored Feb 08, 2012 by Julius Plenz
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fcntl_helpers.c fcntl_helpers.c +5 -0

nocache.c nocache.c +22 -0

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--- a/fcntl_helpers.c
+++ b/fcntl_helpers.c
@@ -29,3 +29,8 @@ void sync_if_writable(int fd)
    if((r & O_ACCMODE) != O_RDONLY)
        fdatasync(fd);
 }
+
+int fcntl_dupfd(int fd, int arg)
+{
+    return fcntl(fd, F_DUPFD, arg);
+}
--- a/nocache.c
+++ b/nocache.c
@@ -19,6 +19,7 @@ int (*_original_close)(int fd);
 static void init(void) __attribute__((constructor));
 static void destroy(void) __attribute__((destructor));
 static void init_mutex(void);
+static void handle_stdout(void);

 int open(const char *pathname, int flags, mode_t mode);
 int open64(const char *pathname, int flags, mode_t mode)
@@ -42,6 +43,7 @@ extern int fadv_dontneed(int fd, off_t offset, off_t len);
 extern int fadv_noreuse(int fd, off_t offset, off_t len);
 extern int valid_fd(int fd);
 extern void sync_if_writable(int fd);
+extern int fcntl_dupfd(int fd, int arg);

 #define _MAX_FDS 1024

@@ -71,6 +73,7 @@ static void init(void)
    for(i = 0; i < _MAX_FDS; i++)
        fds[i].fd = -1;
    init_mutex();
+    handle_stdout();
 }

 static void init_mutex(void)
@@ -80,6 +83,23 @@ static void init_mutex(void)
    pthread_atfork(NULL, NULL, init_mutex);
 }

+/* duplicate stdout if it is a regular file. We will use this later to
+ * fadvise(DONTNEED) on it, although the real stdout was already
+ * closed. This makes "nocache tar cfz" work properly. */
+static void handle_stdout(void)
+{
+    int fd;
+    struct stat st;
+
+    if(fstat(STDOUT_FILENO, &st) == -1 || !S_ISREG(st.st_mode))
+        return;
+
+    fd = fcntl_dupfd(STDOUT_FILENO, 23);
+    if(fd == -1)
+        return;
+    store_pageinfo(fd);
+}
+
 /* try to advise fds that were not manually closed */
 static void destroy(void)
 {
@@ -88,6 +108,8 @@ static void destroy(void)
    for(i = 0; i < _MAX_FDS; i++) {
        if(fds[i].fd == -1)
            continue; /* slot is empty */
+        if(!valid_fd(fds[i].fd))
+            continue;
        pthread_mutex_unlock(&lock);
        free_unclaimed_pages(fds[i].fd);
        pthread_mutex_lock(&lock);