On MacOS, always wrap "dtrace" invocation in "arch -arch $native_arch" (

#304)

.gitignore

@@ -5,3 +5,5 @@ perf*
 
 # IntelliJ IDE files
 .idea/
+# MacOS desktop layout files
+.DS_Store

src/lib.rs

@@ -131,6 +131,36 @@ mod arch {
     #[cfg(target_os = "windows")]
     pub const BLONDIE_ERROR: &str = "could not find dtrace and could not profile using blondie";
 
+    #[cfg(target_os = "macos")]
+    fn base_dtrace_command(sudo: Option<Option<&str>>) -> Command {
+        // If DTrace is spawned from a parent process (or grandparent process etc.) running in Rosetta-emulated x86 mode
+        // on an ARM mac, it will fail to trace the child process with a confusing syntax error in its stdlib .d file.
+        // If the flamegraph binary, or the cargo binary, have been compiled as x86, this can cause all tracing to fail.
+        // To work around that, we unconditionally wrap dtrace on MacOS in the "arch -64/-32" wrapper so it's always
+        // running in the native architecture matching the bit width (32 oe 64) with which "flamegraph" was compiled.
+        // NOTE that dtrace-as-x86 won't trace a deliberately-cross-compiled x86 binary running under Rosetta regardless
+        // of "arch" wrapping; attempts to do that will fail with "DTrace cannot instrument translated processes".
+        // NOTE that using the ARCHPREFERENCE environment variable documented here
+        // (https://www.unix.com/man-page/osx/1/arch/) would be a much simpler solution to this issue, but it does not
+        // seem to have any effect on dtrace when set (via Command::env, shell export, or std::env in the spawning
+        // process).
+        let mut command = sudo_command("arch", sudo);
+
+        #[cfg(target_pointer_width = "64")]
+        command.arg("-64".to_string());
+        #[cfg(target_pointer_width = "32")]
+        command.arg("-32".to_string());
+
+        command.arg(env::var("DTRACE").unwrap_or_else(|_| "dtrace".to_string()));
+        command
+    }
+
+    #[cfg(not(target_os = "macos"))]
+    fn base_dtrace_command(sudo: Option<Option<&str>>) -> Command {
+        let dtrace = env::var("DTRACE").unwrap_or_else(|_| "dtrace".to_string());
+        sudo_command(&dtrace, sudo)
+    }
+
     pub(crate) fn initial_command(
         workload: Workload,
         sudo: Option<Option<&str>>,
@@ -139,8 +169,7 @@ mod arch {
         verbose: bool,
         ignore_status: bool,
     ) -> Option<String> {
-        let dtrace = env::var("DTRACE").unwrap_or_else(|_| "dtrace".to_string());
-        let mut command = sudo_command(&dtrace, sudo);
+        let mut command = base_dtrace_command(sudo);
 
         let dtrace_script = custom_cmd.unwrap_or(format!(
             "profile-{} /pid == $target/ \
@@ -172,7 +201,7 @@ mod arch {
 
                 #[cfg(target_os = "windows")]
                 {
-                    let mut help_test = Command::new(&dtrace);
+                    let mut help_test = crate::arch::base_dtrace_command(None);
 
                     let dtrace_found = help_test
                         .arg("--help")