Address #229: Improve documentation and error output on MacOS when no…

…t running as root (#303)

.gitignore

@@ -2,3 +2,6 @@
 **/*.rs.bk
 *svg
 perf*
+
+# IntelliJ IDE files
+.idea/

README.md

@@ -182,8 +182,9 @@ echo -1 | sudo tee /proc/sys/kernel/perf_event_paranoid
 ### DTrace on macOS
 
 On macOS, there is no alternative to running as superuser in order to
-enable dtrace. The simplest way is to use `--root`, this way `rustc`
-will be run normally but the binary will get run as superuser.
+enable DTrace. This should be done by invoking `sudo flamegraph ...` or 
+`cargo flamegraph --root ...`. Do not do `sudo cargo flamegraph ...`; 
+this can cause problems due to Cargo's build system being run as root.
 
 Be aware that if the binary being tested is user-aware, this does
 change its behaviour.

src/bin/cargo-flamegraph.rs

@@ -459,6 +459,13 @@ fn main() -> anyhow::Result<()> {
         Vec::new()
     };
 
+    #[cfg(target_os = "macos")]
+    if let None = opt.graph.root {
+        return Err(anyhow!(
+            "DTrace requires elevated permissions on MacOS; re-invoke using 'cargo flamegraph --root ...'",
+        ));
+    }
+
     let artifacts = build(&opt, kind)?;
     let workload = workload(&opt, &artifacts)?;
     flamegraph::generate_flamegraph_for_workload(Workload::Command(workload), opt.graph)

src/lib.rs

@@ -447,7 +447,7 @@ pub struct Options {
 
     /// Run with root privileges (using `sudo`). Accepts an optional argument containing command line options which will be passed to sudo
     #[clap(long, value_name = "SUDO FLAGS")]
-    root: Option<Option<String>>,
+    pub root: Option<Option<String>>,
 
     /// Sampling frequency
     #[clap(short = 'F', long = "freq")]