From 28888bbf9f43ec3626bdc1064bd706cdd06f9b1d Mon Sep 17 00:00:00 2001
From: "Jeffrey C. Ollie" <jeff@ocjtech.us>
Date: Mon, 29 Jul 2024 10:47:10 -0500
Subject: [PATCH] make systemd launch detection smarter

---
 src/os/systemd.zig | 51 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 45 insertions(+), 6 deletions(-)

diff --git a/src/os/systemd.zig b/src/os/systemd.zig
index e4b9c479c..22aec57f7 100644
--- a/src/os/systemd.zig
+++ b/src/os/systemd.zig
@@ -1,6 +1,12 @@
 const std = @import("std");
 const builtin = @import("builtin");
 
+const log = std.log.scoped(.systemd);
+
+const c = @cImport({
+    @cInclude("unistd.h");
+});
+
 /// Returns true if the program was launched as a systemd service.
 ///
 /// On Linux, this returns true if the program was launched as a systemd
@@ -9,12 +15,45 @@ const builtin = @import("builtin");
 /// For other platforms and app runtimes, this returns false.
 pub fn launchedBySystemd() bool {
     return switch (builtin.os.tag) {
-        // On Linux, systemd sets the `INVOCATION_ID` (v232+) and the
-        // `JOURNAL_STREAM` (v231+) enviroment variables. If these environment
-        // variables are present (no matter the value) we were launched by
-        // systemd. This can be fooled if Ghostty is launched from another
-        // terminal that does not clean up these environment variables.
-        .linux => std.posix.getenv("INVOCATION_ID") != null and std.posix.getenv("JOURNAL_STREAM") != null,
+        .linux => linux: {
+            // On Linux, systemd sets the `INVOCATION_ID` (v232+) and the
+            // `JOURNAL_STREAM` (v231+) enviroment variables. If these
+            // environment variables are not present we were not launched by
+            // systemd.
+
+            if (std.posix.getenv("INVOCATION_ID") == null) break :linux false;
+            if (std.posix.getenv("JOURNAL_STREAM") == null) break :linux false;
+
+            // If `INVOCATION_ID` and `JOURNAL_STREAM` are present, check to make sure
+            // that our parent process is actually `systemd`, not some other terminal
+            // emulator that doesn't clean up those environment variables.
+
+            const ppid = c.getppid();
+
+            // If the parent PID is 1 we'll assume that it's `systemd` as other init systems
+            // are unlikely.
+
+            if (ppid == 1) break :linux true;
+
+            // If the parent PID is not 1 we need to check to see if we were launched by
+            // a user systemd daemon. Do that by checking the `/proc/<ppid>/exe` symlink
+            // to see if it ends with `/systemd`.
+
+            var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+            const path = std.fmt.bufPrint(&path_buf, "/proc/{d}/exe", .{ppid}) catch {
+                log.err("unable to format path to exe {d}", .{ppid});
+                break :linux false;
+            };
+            var link_buf: [std.fs.max_path_bytes]u8 = undefined;
+            const link = std.fs.readLinkAbsolute(path, &link_buf) catch {
+                log.err("unable to read link '{s}'", .{path});
+                break :linux false;
+            };
+
+            if (std.mem.endsWith(u8, link, "/systemd")) break :linux true;
+
+            break :linux false;
+        },
 
         // No other system supports systemd so always return false.
         else => false,