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Add %p to LLVM_PROFILE_FILE pattern when running tests with coverage #8894

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Add %p to LLVM_PROFILE_FILE pattern when running tests with coverage #8894

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25 changes: 19 additions & 6 deletions Sources/Commands/Utilities/TestingSupport.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -197,12 +197,25 @@ enum TestingSupport {
if buildParameters.testingParameters.enableCodeCoverage {
// Defines the path at which the profraw files will be written on test execution.
//
// `%m` will create a pool of profraw files and append the data from
// each execution in one of the files. This doesn't matter for serial
// execution but is required when the tests are running in parallel as
// SwiftPM repeatedly invokes the test binary with the test case name as
// the filter.
let codecovProfile = buildParameters.buildPath.appending(components: "codecov", "\(library)%m.profraw")
// `%Nm` will create a pool of N profraw files and append the data from each execution
// in one of the files. The runtime takes care of selecting a raw profile from the pool,
// locking it, and updating it before the program exits. If N is not specified, it is
// inferred to be 1.
//
// This is fine for parallel execution within a process, but for parallel tests, SwiftPM
// repeatedly invokes the test binary with the testcase name as the filter and the
// locking cannot be enforced by the runtime across the process boundaries.
//
// It's also possible that tests themselves will fork (e.g. for exit tests provided by
// Swift Testing), which will inherit the environment of the parent process, and so
// write to the same file, leading to profile data corruption.
//
// For these reasons, we unilaterally also add a %p, which will cause uniquely named
// files per process.
//
// These are all merged using `llvm-profdata merge` once the outer test command has
// completed.
let codecovProfile = buildParameters.buildPath.appending(components: "codecov", "\(library)%m.%p.profraw")
env["LLVM_PROFILE_FILE"] = codecovProfile.pathString
}
#if !os(macOS)
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