From b0f19c2af45718f8eb27638a1135c3cc03a92ec9 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Fri, 22 Jan 2021 19:23:36 +0100
Subject: [PATCH] LibC: Templatize unique filename enumeration for mkstemp() et
 al

This allows us to implement mkstemp() with open() directly, instead of
first lstat()'ing, and then open()'ing the filename.

Also implement tmpfile() in terms of mkstemp() instead of mktemp().
---
 Userland/Libraries/LibC/stdio.cpp  |  7 +---
 Userland/Libraries/LibC/stdlib.cpp | 61 ++++++++++++++++++------------
 2 files changed, 38 insertions(+), 30 deletions(-)

diff --git a/Userland/Libraries/LibC/stdio.cpp b/Userland/Libraries/LibC/stdio.cpp
index 1338b82f4e..6411df4d79 100644
--- a/Userland/Libraries/LibC/stdio.cpp
+++ b/Userland/Libraries/LibC/stdio.cpp
@@ -1205,16 +1205,11 @@ void funlockfile([[maybe_unused]] FILE* filehandle)
 FILE* tmpfile()
 {
     char tmp_path[] = "/tmp/XXXXXX";
-    if (__generate_unique_filename(tmp_path) < 0)
-        return nullptr;
-
-    int fd = open(tmp_path, O_CREAT | O_EXCL | O_RDWR, S_IWUSR | S_IRUSR);
+    int fd = mkstemp(tmp_path);
     if (fd < 0)
         return nullptr;
-
     // FIXME: instead of using this hack, implement with O_TMPFILE or similar
     unlink(tmp_path);
-
     return fdopen(fd, "rw");
 }
 }
diff --git a/Userland/Libraries/LibC/stdlib.cpp b/Userland/Libraries/LibC/stdlib.cpp
index 47dd242bb0..8d2f0911b0 100644
--- a/Userland/Libraries/LibC/stdlib.cpp
+++ b/Userland/Libraries/LibC/stdlib.cpp
@@ -171,14 +171,13 @@ static bool is_either(char* str, int offset, char lower, char upper)
     return ch == lower || ch == upper;
 }
 
-__attribute__((warn_unused_result)) int __generate_unique_filename(char* pattern)
+template<typename Callback>
+inline int generate_unique_filename(char* pattern, Callback callback)
 {
     size_t length = strlen(pattern);
 
-    if (length < 6 || memcmp(pattern + length - 6, "XXXXXX", 6)) {
-        errno = EINVAL;
-        return -1;
-    }
+    if (length < 6 || memcmp(pattern + length - 6, "XXXXXX", 6))
+        return EINVAL;
 
     size_t start = length - 6;
 
@@ -187,13 +186,11 @@ __attribute__((warn_unused_result)) int __generate_unique_filename(char* pattern
     for (int attempt = 0; attempt < 100; ++attempt) {
         for (int i = 0; i < 6; ++i)
             pattern[start + i] = random_characters[(arc4random() % (sizeof(random_characters) - 1))];
-        struct stat st;
-        int rc = lstat(pattern, &st);
-        if (rc < 0 && errno == ENOENT)
+        if (callback() == IterationDecision::Break)
             return 0;
     }
-    errno = EEXIST;
-    return -1;
+
+    return EEXIST;
 }
 
 extern "C" {
@@ -727,31 +724,47 @@ int system(const char* command)
 
 char* mktemp(char* pattern)
 {
-    if (__generate_unique_filename(pattern) < 0)
+    auto error = generate_unique_filename(pattern, [&] {
+        struct stat st;
+        int rc = lstat(pattern, &st);
+        if (rc < 0 && errno == ENOENT)
+            return IterationDecision::Break;
+        return IterationDecision::Continue;
+    });
+    if (error) {
         pattern[0] = '\0';
-
+        errno = error;
+    }
     return pattern;
 }
 
 int mkstemp(char* pattern)
 {
-    char* path = mktemp(pattern);
-
-    int fd = open(path, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR); // I'm using the flags I saw glibc using.
-    if (fd >= 0)
-        return fd;
-
-    return -1;
+    int fd = -1;
+    auto error = generate_unique_filename(pattern, [&] {
+        fd = open(pattern, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR); // I'm using the flags I saw glibc using.
+        if (fd >= 0)
+            return IterationDecision::Break;
+        return IterationDecision::Continue;
+    });
+    if (error) {
+        errno = error;
+        return -1;
+    }
+    return fd;
 }
 
 char* mkdtemp(char* pattern)
 {
-    if (__generate_unique_filename(pattern) < 0)
+    auto error = generate_unique_filename(pattern, [&] {
+        if (mkdir(pattern, 0700) == 0)
+            return IterationDecision::Break;
+        return IterationDecision::Continue;
+    });
+    if (error) {
+        errno = error;
         return nullptr;
-
-    if (mkdir(pattern, 0700) < 0)
-        return nullptr;
-
+    }
     return pattern;
 }