From 910ae5a99d7aeee8c8feac92d9a08e55717fa2c3 Mon Sep 17 00:00:00 2001
From: Ilia Ross <ilia@ross.gdn>
Date: Thu, 8 Aug 2024 15:44:52 +0300
Subject: [PATCH] Add better support in case of a race condition

https://github.com/webmin/authentic-theme/commit/c3098fe01a8cd4c9060c6af5c514245f5904a377#r145157572
---
 stats-lib-funcs.pl | 83 +++++++++++++++++++++++++++++-----------------
 1 file changed, 52 insertions(+), 31 deletions(-)

diff --git a/stats-lib-funcs.pl b/stats-lib-funcs.pl
index 1488af9b6..bfd508b2b 100644
--- a/stats-lib-funcs.pl
+++ b/stats-lib-funcs.pl
@@ -5,7 +5,7 @@
 #
 use strict;
 use feature 'state';
-use Fcntl ':flock';
+use Fcntl qw(:flock O_RDWR O_CREAT);
 
 our $json;
 eval "use JSON::XS";
@@ -78,38 +78,59 @@ sub stats_read_file_contents {
 # Write file contents
 sub stats_write_file_contents {
     my ($filename, $contents) = @_;
-    # Open file for writing
-    my $fh;
-    if (!open($fh, '>', $filename)) {
-        error_stderr("Failed to open file '$filename' for writing: $!");
-        return 0;
-    }
-    # Acquire lock the file for writing
-    if (!flock($fh, LOCK_EX)) {
-        error_stderr("Failed to acquire exclusive lock on file '$filename': $!");
-        close($fh);
+    my $cleanup = sub {
+        my ($fh, $unlock, $success) = @_;
+        # Unlock the file. Not strictly necessary as Perl
+        # automatically releases locks on file closure
+        flock($fh, LOCK_UN) if ($unlock);
+        # Close file handle
+        close($fh)
+            or do {
+                error_stderr("Failed to close file '$filename': $!");
+                undef($fh);
+                return 0;
+            };
+        # Release memory. Generally not necessary as Perl's garbage collection
+        # should always handle this. However, it doesn't hurt and can be
+        # useful in certain scenarios, like long-running processes
         undef($fh);
-        return 0;
-    }
+        # Return result
+        return $success ? 1 : 0;
+    };
+    # Open file for reading and writing without truncating it, or 
+    # create it if it doesn't exist
+    sysopen(my $fh, $filename, O_RDWR | O_CREAT)
+        or do {
+            error_stderr("Failed to open file '$filename' for reading and writing: $!");
+            return 0;
+        };
+    # Acquire exclusive lock on the file for writing
+    flock($fh, LOCK_EX)
+        or do {
+            error_stderr("Failed to acquire exclusive lock on file '$filename': $!");
+            return $cleanup->($fh, 0);
+        };
+    # Truncate the file after acquiring the lock
+    truncate($fh, 0)
+        or do {
+            error_stderr("Failed to truncate file '$filename': $!");
+            return $cleanup->($fh, 1);
+        };
+    # Reset the file pointer to the beginning of the file
+    # preventing potential race conditions
+    seek($fh, 0, 0)
+        or do {
+            error_stderr("Failed to seek to the beginning of file '$filename': $!");
+            return $cleanup->($fh, 1);
+        };
     # Write to file
-    if (!print($fh $contents)) {
-        error_stderr("Failed to write to file '$filename': $!");
-        close($fh);
-        undef($fh);
-        return 0;
-    }
-    # Unlock the file
-    flock($fh, LOCK_UN);
-    # Close file
-    if (!close($fh)) {
-        error_stderr("Failed to close file '$filename': $!");
-        undef($fh);
-        return 0;
-    }
-    # Release memory
-    undef($fh);
-    # Return success
-    return 1;
+    print($fh $contents)
+        or do {
+            error_stderr("Failed to write to file '$filename': $!");
+            return $cleanup->($fh, 1);
+        };
+    # Clean up and return success
+    return $cleanup->($fh, 1, 1);
 }
 
 # Check if feature is enabled