fix: panic when perf counter could not be initialized

As reported in iopsystems#14, the CPU CPI sampler will panic if the cycles or
instructions perf counters cannot be initialized.

This PR fixes the bug by returning a Nop sampler if there are
exceptions during initialization.

src/samplers/cpu/cpi/mod.rs

@@ -21,26 +21,49 @@ impl Cpi {
     pub fn new(_config: &Config) -> Self {
         let now = Instant::now();
 
-        let mut group = Group::new().expect("couldn't init perf group");
-        let counters = vec![
-            (
-                Counter::new(&CPU_CYCLES, None),
-                Builder::new()
-                    .group(&mut group)
-                    .kind(Hardware::CPU_CYCLES)
-                    .build()
-                    .expect("failed to init cycles counter"),
-            ),
-            (
-                Counter::new(&CPU_INSTRUCTIONS, None),
-                Builder::new()
-                    .group(&mut group)
-                    .kind(Hardware::INSTRUCTIONS)
-                    .build()
-                    .expect("failed to init instructions counter"),
-            ),
-        ];
-        group.enable().expect("couldn't enable perf counters");
+        // initialize a group for the perf counters
+        let mut group = match Group::new() {
+            Ok(g) => g,
+            Err(_) => {
+                error!("couldn't init perf group");
+                return Nop { };
+            }
+        }
+
+        // initialize the counters
+        let mut counters = vec![];
+
+        match Builder::new()
+        .group(&mut group)
+        .kind(Hardware::CPU_CYCLES)
+        .build() {
+            Ok(counter) => {
+                counters.push((Counter::new(&CPU_CYCLES, None), counter));
+            }
+            Err(_) => {
+                error!("failed to initialize cpu cycles perf counter");
+                return Nop { };
+            }
+        }
+
+        match Builder::new()
+        .group(&mut group)
+        .kind(Hardware::INSTRUCTIONS)
+        .build() {
+            Ok(counter) => {
+                counters.push((Counter::new(&CPU_INSTRUCTIONS, None), counter));
+            }
+            Err(_) => {
+                error!("failed to initialize cpu instructions perf counter");
+                return Nop { };
+            }
+        }
+
+        // enable the counters in the group
+        if group.enable().is_err() {
+            error!("couldn't enable perf counter group");
+            return Nop { };
+        }
 
         Self {
             prev: now,