Show human-readable git errors (#8152)

Show human-readable git errors

### Motivation:

I noticed that when swiftpm cache is empty and there's a network issue,
then `swift build` throws an error which is not helpful, something like
this:

```
error: GitShellError(result: <AsyncProcessResult: exit: terminated(code: 128), output:
 
>)
```

where it does say "output" but doesn't actually include output or show
any details about what dependency caused the error.

### Modifications:

- `GitShellError` now conforms to CustomStringConvertible protocol, so
that when it is thrown then it will print what was the git error
- `GitShelError` has now package access to allow catching it explicitly.

### Result:

Errors now tell details of the failure: 

```
error: Git command 'git -C /Users/yy/Library/Caches/org.swift.swiftpm/repositories/postgres-kit-e4358808 config --get remote.origin.url' failed: fatal: cannot change to '/Users/yy/Library/Caches/org.swift.swiftpm/repositories/postgres-kit-e4358808': No such file or directory
```

---------

Co-authored-by: Max Desiatov <m_desiatov@apple.com>

Author: yyvch

Sources/SourceControl/GitRepository.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift open source project
 //
-// Copyright (c) 2014-2020 Apple Inc. and the Swift project authors
+// Copyright (c) 2014-2024 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See http://swift.org/LICENSE.txt for license information
@@ -1161,8 +1161,16 @@ extension GitFileSystemView: @unchecked Sendable {}
 
 // MARK: - Errors
 
-private struct GitShellError: Error {
+package struct GitShellError: Error, CustomStringConvertible {
     let result: AsyncProcessResult
+
+    public var description: String {
+        let stdout = (try? self.result.utf8Output()) ?? ""
+        let stderr = (try? self.result.utf8stderrOutput()) ?? ""
+        let output = (stdout + stderr).spm_chomp()
+        let command = self.result.arguments.joined(separator: " ")
+        return "Git command '\(command)' failed: \(output)"
+    }
 }
 
 private enum GitInterfaceError: Swift.Error {

Tests/SourceControlTests/GitRepositoryProviderTests.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift open source project
 //
-// Copyright (c) 2022 Apple Inc. and the Swift project authors
+// Copyright (c) 2022-2024 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See http://swift.org/LICENSE.txt for license information
@@ -40,4 +40,80 @@ class GitRepositoryProviderTests: XCTestCase {
             XCTAssertThrowsError(try provider.isValidDirectory(notGitChildPath))
         }
     }
+
+    func testIsValidDirectoryThrowsPrintableError() throws {
+        try testWithTemporaryDirectory { temp in
+            let provider = GitRepositoryProvider()
+            let expectedErrorMessage = "not a git repository"
+            XCTAssertThrowsError(try provider.isValidDirectory(temp)) { error in
+                let errorString = String(describing: error)
+                XCTAssertTrue(
+                    errorString.contains(expectedErrorMessage),
+                    "Error string '\(errorString)' should contain '\(expectedErrorMessage)'"
+                )
+            }
+        }
+    }
+
+    func testGitShellErrorIsPrintable() throws {
+        let stdOut = "An error from Git - stdout"
+        let stdErr = "An error from Git - stderr"
+        let arguments = ["git", "error"]
+        let command = "git error"
+        let result = AsyncProcessResult(
+            arguments: arguments,
+            environment: [:],
+            exitStatus: .terminated(code: 1),
+            output: .success(Array(stdOut.utf8)),
+            stderrOutput: .success(Array(stdErr.utf8))
+        )
+        let error = GitShellError(result: result)
+        let errorString = "\(error)"
+        XCTAssertTrue(
+            errorString.contains(stdOut),
+            "Error string '\(errorString)' should contain '\(stdOut)'"
+        )
+        XCTAssertTrue(
+            errorString.contains(stdErr),
+            "Error string '\(errorString)' should contain '\(stdErr)'"
+        )
+        XCTAssertTrue(
+            errorString.contains(command),
+            "Error string '\(errorString)' should contain '\(command)'"
+        )
+    }
+
+    func testGitShellErrorEmptyStdOut() throws {
+        let stdErr = "An error from Git - stderr"
+        let result = AsyncProcessResult(
+            arguments: ["git", "error"],
+            environment: [:],
+            exitStatus: .terminated(code: 1),
+            output: .success([]),
+            stderrOutput: .success(Array(stdErr.utf8))
+        )
+        let error = GitShellError(result: result)
+        let errorString = "\(error)"
+        XCTAssertTrue(
+            errorString.contains(stdErr),
+            "Error string '\(errorString)' should contain '\(stdErr)'"
+        )
+    }
+
+    func testGitShellErrorEmptyStdErr() throws {
+        let stdOut = "An error from Git - stdout"
+        let result = AsyncProcessResult(
+            arguments: ["git", "error"],
+            environment: [:],
+            exitStatus: .terminated(code: 1),
+            output: .success(Array(stdOut.utf8)),
+            stderrOutput: .success([])
+        )
+        let error = GitShellError(result: result)
+        let errorString = "\(error)"
+        XCTAssertTrue(
+            errorString.contains(stdOut),
+            "Error string '\(errorString)' should contain '\(stdOut)'"
+        )
+    }
 }