From 8dddc62aeb636c302de3876607027c7b48a94ba0 Mon Sep 17 00:00:00 2001
From: Mahmoud Al-Qudsi <mqudsi@neosmart.net>
Date: Sun, 30 Dec 2018 19:39:59 -0600
Subject: [PATCH] Optimize ASSERT_IS_NOT_FORKED_CHILD()
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Use `pthread_atfork()` to mark child processes as dirty when `fork()` is
invoked rather than needing to call into the kernel each time
`ASSERT_IS_NOT_FORKED_CHILD()` is called.

This makes simple test cases that hit `ASSERT_IS_NOT_FORKED_CHILD()` 1.8x faster.

                        ------------------------

With a7998c4829a80269bcd6af941812e35c86bf49e2 reverted but before this optimization:

```
mqudsi@ZBOOK ~/r/fish-shell> hyperfine -S build/fish 'for i in (seq 100000); test 1 = 1; end'
Benchmark #1: for i in (seq 100000); test 1 = 1; end

  Time (mean ± σ):     717.8 ms ±  14.9 ms    [User: 503.4 ms, System: 216.2 ms]
  Range (min … max):   692.3 ms … 740.2 ms
```

With a7998c4829a80269bcd6af941812e35c86bf49e2 reverted and with this optimization:

```
mqudsi@ZBOOK ~/r/fish-shell> hyperfine -S build/fish 'for i in (seq 100000); test 1 = 1; end'
Benchmark #1: for i in (seq 100000); test 1 = 1; end

  Time (mean ± σ):     397.2 ms ±  22.3 ms    [User: 322.1 ms, System: 79.3 ms]
  Range (min … max):   376.0 ms … 444.0 ms
```

Without a7998c4829a80269bcd6af941812e35c86bf49e2 reverted and with this optimization:

mqudsi@ZBOOK ~/r/fish-shell> hyperfine -S build/fish 'for i in (seq 100000); test 1 = 1; end'
Benchmark #1: for i in (seq 100000); test 1 = 1; end

  Time (mean ± σ):     423.4 ms ±  51.6 ms    [User: 363.2 ms, System: 61.3 ms]
  Range (min … max):   378.4 ms … 541.1 ms
```
---
 src/common.cpp | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/src/common.cpp b/src/common.cpp
index 688ed30d2..527210a1f 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -54,7 +54,11 @@ constexpr wint_t NOT_A_WCHAR = static_cast<wint_t>(WEOF);
 
 struct termios shell_modes;
 
+/// This allows us to determine if we're running on the main thread
 static std::atomic<size_t> thread_id { 0 };
+/// This allows us to notice when we've forked.
+static bool is_forked_proc = false;
+/// This allows us to bypass the main thread checks
 static bool thread_asserts_cfg_for_testing = false;
 
 wchar_t ellipsis_char;
@@ -2171,20 +2175,13 @@ void set_main_thread() {
 void configure_thread_assertions_for_testing() { thread_asserts_cfg_for_testing = true; }
 
 bool is_forked_child() {
-    // Just bail if nobody's called setup_fork_guards, e.g. some of our tools.
-    if (!initial_pid) return false;
-
-    bool is_child_of_fork = (getpid() != initial_pid);
-    if (is_child_of_fork) {
-        debug(0, L"Uh-oh: getpid() != initial_pid: %d != %d\n", getpid(), initial_pid);
-        while (1) sleep(10000);
-    }
-    return is_child_of_fork;
+    return is_forked_proc;
 }
 
 void setup_fork_guards() {
-    // Notice when we fork by stashing our pid. This seems simpler than pthread_atfork().
-    initial_pid = getpid();
+    pthread_atfork(nullptr, nullptr, []() {
+        is_forked_proc = true;
+    });
 }
 
 void save_term_foreground_process_group() {