Include diagnosing error messages for userevents metrics exporter (#1273)

Author: lzchen

opentelemetry-user-events-metrics/CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Changed
 
+- Include error diagnosing messages for registering tracepoint
+    [#1273](https://github.com/open-telemetry/opentelemetry-rust/pull/1273).
 - Add version, protocol to schema
     [#1224](https://github.com/open-telemetry/opentelemetry-rust/pull/1224).
 

opentelemetry-user-events-metrics/src/tracepoint/mod.rs

@@ -1,5 +1,7 @@
 use core::ffi;
 use eventheader::_internal as ehi;
+use opentelemetry::{global, metrics::MetricsError};
+use std::panic;
 use std::pin::Pin;
 
 /// Protocol constant
@@ -68,8 +70,7 @@ pub fn write(trace_point: &ehi::TracepointState, buffer: &[u8]) -> i32 {
 /// Requires: this tracepoint is not currently registered.
 /// The tracepoint must be in a Pin<&TracepointState> because we must ensure it will never be moved
 ///
-/// Return value is 0 for success or an errno code for error. The return value is
-/// provided to help with debugging and should usually be ignored in release builds.
+/// Return value is 0 for success or -1 for failed register.
 ///
 /// # Safety
 ///
@@ -79,5 +80,38 @@ pub unsafe fn register(trace_point: Pin<&ehi::TracepointState>) -> i32 {
     debug_assert!(METRICS_EVENT_DEF[METRICS_EVENT_DEF.len() - 1] == b'\0');
 
     // CStr::from_bytes_with_nul_unchecked is ok because METRICS_EVENT_DEF ends with "\0".
-    trace_point.register(ffi::CStr::from_bytes_with_nul_unchecked(METRICS_EVENT_DEF))
+    // Returns errno code 95 if trace/debug file systems are not mounted
+    // Returns errno code 13 if insufficient permissions
+    // If tracepoint doesn't exist, it will create one automatically
+    let result = panic::catch_unwind(|| {
+        // CStr::from_bytes_with_nul_unchecked is ok because METRICS_EVENT_DEF ends with "\0".
+        trace_point.register(ffi::CStr::from_bytes_with_nul_unchecked(METRICS_EVENT_DEF))
+    });
+
+    match result {
+        Ok(value) => {
+            if value == 0 {
+                // Temporary print as a measure for quick testing
+                // will be replaced with proper logging mechanism
+                println!("Tracepoint registered successfully.")
+            } else if value == 95 {
+                global::handle_error(MetricsError::Other(
+                    "Trace/debug file systems are not mounted.".into(),
+                ));
+            } else if value == 13 {
+                global::handle_error(MetricsError::Other(
+                    "Insufficient permissions. You need read/write/execute permissions to user_events tracing directory.".into(),
+                ));
+            }
+            value
+        }
+        // We don't want to ever panic so we catch the error and return a unique code for retry
+        Err(err) => {
+            global::handle_error(MetricsError::Other(format!(
+                "Tracepoint failed to register: {:?}.",
+                err,
+            )));
+            -1
+        }
+    }
 }