cppcheck: warn on use of flock

.cppcheck.rules

@@ -1,5 +1,6 @@
 <?xml version="1.0"?>
 
+<![CDATA[
 <!-- Sadly we can't enable the following two rules since doing so causes false
      positives in standard header files rather than just project specific
      source files. If we can find a way to enable these rules by also
@@ -22,3 +23,13 @@
     </message>
 </rule>
 <--!>
+]]>
+
+<rule>
+    <pattern>flock \(</pattern>
+    <message>
+        <id>flockSemanticsWarning</id>
+        <severity>warning</severity>
+        <summary>flock has a fallback implemented in terms of fcntl; ensure that the fcntl semantics will apply (see http://0pointer.de/blog/projects/locking.html)</summary>
+    </message>
+</rule>

src/env_universal_common.cpp

@@ -595,6 +595,9 @@ bool env_universal_t::open_and_acquire_lock(const wcstring &path, int *out_fd) {
         // Try taking the lock, if necessary. If we failed, we may be on lockless NFS, etc.; in that
         // case we pretend we succeeded. See the comment in save_to_path for the rationale.
         if (needs_lock) {
+            // This is safe in terms of the fallback function implemented in terms of fcntl: only ever
+            // run on the main thread, and protected by the universal variable lock
+            // cppcheck-suppress flockSemanticsWarning
             while (flock(fd, LOCK_EX) < 0) {
                 /* error */
                 if (errno != EINTR) {

src/fallback.h

@@ -129,6 +129,9 @@ int killpg(int pgr, int sig);
 /// Fallback implementation of flock in terms of fcntl
 /// Danger! The semantics of flock and fcntl locking are very different.
 /// Use with caution.
+// Ignore the cppcheck warning as this is the implementation that it is
+// warning about!
+// cppcheck-suppress flockSemanticsWarning
 int flock(int fd, int op);
 
 #define LOCK_SH 1       /* Shared lock.  */