wsd: improved logging of thread renaming

The thread name helps one track threads in logs.
When renaming threads it's important to log the process
and previous thread name (if any), so grepping is more
fruitful and tracking is easier.

Change-Id: I47a948d77629b387cc1e9fd58fdd88e1ae1168df
Reviewed-on: https://gerrit.libreoffice.org/79327
Reviewed-by: Ashod Nakashian <[email protected]>
Tested-by: Ashod Nakashian <[email protected]>

common/Util.cpp

@@ -530,19 +530,30 @@ namespace Util
         return replace(r, "\n", " / ");
     }
 
+    // prctl(2) supports names of up to 16 characters, including null-termination.
+    // Although in practice on linux more than 16 chars is supported.
     static thread_local char ThreadName[32] = {0};
+    static_assert(sizeof(ThreadName) >= 16, "ThreadName should have a statically known size, and not be a pointer.");
 
     void setThreadName(const std::string& s)
     {
-        strncpy(ThreadName, s.c_str(), 31);
-        ThreadName[31] = '\0';
+        // Copy the current name.
+        const std::string knownAs
+            = ThreadName[0] ? "known as [" + std::string(ThreadName) + "]" : "unnamed";
+
+        // Set the new name.
+        strncpy(ThreadName, s.c_str(), sizeof(ThreadName) - 1);
+        ThreadName[sizeof(ThreadName) - 1] = '\0';
 #ifdef __linux
         if (prctl(PR_SET_NAME, reinterpret_cast<unsigned long>(s.c_str()), 0, 0, 0) != 0)
-            LOG_SYS("Cannot set thread name of " << getThreadId() << " (" << std::hex <<
-                    std::this_thread::get_id() << std::dec << ") to [" << s << "].");
+            LOG_SYS("Cannot set thread name of "
+                    << getThreadId() << " (" << std::hex << std::this_thread::get_id() << std::dec
+                    << ") of process " << getpid() << " currently " << knownAs << " to [" << s
+                    << "].");
         else
-            LOG_INF("Thread " << getThreadId() << " (" << std::hex <<
-                    std::this_thread::get_id() << std::dec << ") is now called [" << s << "].");
+            LOG_INF("Thread " << getThreadId() << " (" << std::hex << std::this_thread::get_id()
+                              << std::dec << ") of process " << getpid() << " formerly " << knownAs
+                              << " is now called [" << s << "].");
 #elif defined IOS
         [[NSThread currentThread] setName:[NSString stringWithUTF8String:ThreadName]];
         LOG_INF("Thread " << getThreadId() <<
@@ -556,13 +567,14 @@ namespace Util
         if (ThreadName[0] == '\0')
         {
 #ifdef __linux
+            // prctl(2): The buffer should allow space for up to 16 bytes; the returned string will be null-terminated.
             if (prctl(PR_GET_NAME, reinterpret_cast<unsigned long>(ThreadName), 0, 0, 0) != 0)
                 strncpy(ThreadName, "<noid>", sizeof(ThreadName) - 1);
 #elif defined IOS
             const char *const name = [[[NSThread currentThread] name] UTF8String];
-            strncpy(ThreadName, name, 31);
-            ThreadName[31] = '\0';
+            strncpy(ThreadName, name, sizeof(ThreadName) - 1);
 #endif
+            ThreadName[sizeof(ThreadName) - 1] = '\0';
         }
 
         // Avoid so many redundant system calls