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Supporting relative paths: XCTest failures in Xcode

If you build your iOS app with an alternate build system such as bazel, it's likely that you use relative paths, instead of absolute paths, for compilation.

Specifically, when building swift code, Xcode calls the compiler with something like:

swiftc [ARGS] /path/to/srcroot/path/to/file1.swift /path/to/srcroot/path/to/file2.swift

Where bazel will call the compiler with something like:

swiftc [ARGS] path/to/file1.swift path/to/file2.swift

Normally, this difference is inconsequential, both compilations will result in a similar enough output. So, the question is: Why would you pick one over the other? For bazel, the answer lies in its core feature of "hermeticity". In bazel's case, being hermetic means that given the same inputs you always produce the same outputs. This means that regardless of what machine you're building on, or what directory your source is cloned in, the results should be the same. Because, those details aren't considered important inputs in the build.

Unfortunately, in a few places, Xcode relies on paths being absolute. Today, we'll look at how Xcode reports test failures in the UI. Specifically the underlying XCTIssue that XCTest creates is expected to be instantiated with an absolute path. This absolute path is populated from the #filePath (previously #file) keyword which is supposed to reference the absolute path of the current source file.

The first question is: How does the Swift compiler know what the absolute path of the current file is? It's easy when Xcode passes an absolute path to the compiler. But, what if you pass a relative path? In this case, the Swift compiler uses the directory passed with the -working-directory argument to make the path absolute. It turns out if you don't pass this argument, the compiler has no choice but to use the relative path. This means the #filePath keyword ends up translating to a relative path, which means the XCTIssue is created with a relative path.

With relative paths when you run your tests in Xcode and they fail, clicking the failure in the issue navigator doesn't do anything. But, it's supposed to jump you to the test case that failed (FB8451256, FB8454623).

So, how do we fix this? Luckily, we know the core issue is XCTIssue is created with a relative path. Since XCTIssue instances are created as part of our process, we can swizzle it to fix this.

Looking at the underlying XCTSourceCodeLocation docs, we can see there are 2 initializers we're potentially interested in. Setting some quick breakpoints we can see that XCTIssue goes through the init(fileURL:lineNumber:) initializer. When we inspect the argument it receives in the debugger, we can see it's the relative path we passed to the compiler. Knowing this, we can surmise that by swizzling the initializer, and make the argument an absolute path, we can satisfy Xcode's requirement. So, what path do we use? Using Xcode's scheme environment variables, we can pass Xcode's SRCROOT through an environment variable named the same thing:

Make sure to have the "Expand Variables Based On" dropdown set to some target (FB8454879) or the $(SRCROOT) string will be passed through literally.

Now, for the swizzling:

import Foundation
import XCTest

// NOTE: This path has to start with a / for fileURLWithPath to resolve it correctly as an absolute path
public let kSourceRoot = ProcessInfo.processInfo.environment["SRCROOT"]!

private func remapFileURL(_ fileURL: URL) -> URL {
    if fileURL.path.hasPrefix(kSourceRoot) {
        return fileURL
    }

    return URL(fileURLWithPath: "\(kSourceRoot)/\(fileURL.relativePath)")
}

private extension XCTSourceCodeLocation {
    @objc
    convenience init(initWithRelativeFileURL relativeURL: URL, lineNumber: Int) {
        // NOTE: This call is not recursive because of swizzling
        self.init(initWithRelativeFileURL: remapFileURL(relativeURL), lineNumber: lineNumber)
    }
}

func swizzleXCTSourceCodeLocationIfNeeded() {
    // NOTE: Make sure our "Expand Variables Based On" is set correctly
    if kSourceRoot == "$(SRCROOT)" {
        fatalError("Got unsubstituted SRCROOT")
    }

    let originalSelector = #selector(XCTSourceCodeLocation.init(fileURL:lineNumber:))
    let swizzledSelector = #selector(XCTSourceCodeLocation.init(initWithRelativeFileURL:lineNumber:))

    guard let originalMethod = class_getInstanceMethod(XCTSourceCodeLocation.self, originalSelector),
        let swizzledMethod = class_getInstanceMethod(XCTSourceCodeLocation.self, swizzledSelector) else
    {
        fatalError("Failed to swizzle XCTSourceCodeLocation")
    }

    method_exchangeImplementations(originalMethod, swizzledMethod)
}

With this implementation, you need to call swizzleXCTSourceCodeLocationIfNeeded() somewhere. Using the NSPrincipalClass plist key, we can define a class that is initialized as soon as your test bundle starts to run. We can define a small class to call this:

import ObjectiveC

final class UnitTestMain: NSObject {
    override init() {
        super.init()

        swizzleXCTSourceCodeLocationIfNeeded()
    }
}

Disclaimer: this relies on a lot of implementation details in Xcode which might break in the future. You should avoid this if possible.