Fix Bug That Truncates Stack Traces

[richard-tunein]

What It Fixes

This PR updates takeStacktrace() to fix some logic errors. The errors would cause that function to improperly truncate a stack trace in many cases.

Details

	programCounters := _stacktracePool.Get().(*programCounters)
	// ...
	for {
		n := runtime.Callers(2, programCounters.pcs)
		if n < cap(programCounters.pcs) {
			programCounters.pcs = programCounters.pcs[:n]
			break
		}
	}

The above code seems to assume that runtime.Callers() writes up to the capacity of the slice, when in fact it will only write up to the length of the slice. This would work the first time that a variable was allocated by the pool, since the slice's capacity and length would be equal. However, the code then re-sizes the slice to reflect the number of items written. When the data structure is put back into the pool, the slice will now have a smaller length.

The next time that data structure is re-used from the pool, runtime.Callers() will only write up to and return that smaller length. If the number of frames is greater than the length, the n < cap(programCounters.pcs) condition will still return true, and the loop will break without obtaining all of the items available.

How I Fixed It

The fix avoids re-slicing programCounters.pcs. Instead, we record the number of total frames in a numFrames variable outside of the for loop, and use it to provide a new (sub-)slice as a parameter to the runtime.CallersFrames() function.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #477 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #477   +/-   ##
=======================================
  Coverage   97.12%   97.12%           
=======================================
  Files          36       36           
  Lines        1912     1912           
=======================================
  Hits         1857     1857           
  Misses         45       45           
  Partials       10       10

Impacted Files	Coverage Δ
stacktrace.go	86.11% <100%> (ø)	⬆️

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[akshayjshah]

This is great - thanks for the fix, and for the detailed PR message!