Erase environment variables set by exit tests after reading them. (#1044)

The environment variables we currently use to pass information from the
parent process to the child process are implementation details of exit
tests and may change or be removed in a future update. To minimize the
risk of code relying on these environment variables, and to avoid
accidentally trying to open inherited file descriptors twice, clear the
variables after reading them.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ExitTests/ExitTest.swift

@@ -457,6 +457,10 @@ extension ExitTest {
       return nil
     }
 
+    // Erase the environment variable so that it cannot accidentally be opened
+    // twice (nor, in theory, affect the code of the exit test.)
+    Environment.setVariable(nil, named: "SWT_EXPERIMENTAL_BACKCHANNEL")
+
     var fd: CInt?
 #if SWT_TARGET_OS_APPLE || os(Linux) || os(FreeBSD) || os(OpenBSD)
     fd = CInt(backChannelEnvironmentVariable)
@@ -487,6 +491,10 @@ extension ExitTest {
     // Find the ID of the exit test to run, if any, in the environment block.
     var id: ExitTest.ID?
     if var idString = Environment.variable(named: "SWT_EXPERIMENTAL_EXIT_TEST_ID") {
+      // Clear the environment variable. It's an implementation detail and exit
+      // test code shouldn't be dependent on it. Use ExitTest.current if needed!
+      Environment.setVariable(nil, named: "SWT_EXPERIMENTAL_EXIT_TEST_ID")
+
       id = try? idString.withUTF8 { idBuffer in
         try JSON.decode(ExitTest.ID.self, from: UnsafeRawBufferPointer(idBuffer))
       }

Sources/Testing/Support/Environment.swift

@@ -235,3 +235,42 @@ enum Environment {
     }
   }
 }
+
+// MARK: - Setting variables
+
+extension Environment {
+  /// Set the environment variable with the specified name.
+  ///
+  /// - Parameters:
+  ///   - value: The new value for the specified environment variable. Pass
+  ///     `nil` to remove the variable from the current process' environment.
+  ///   - name: The name of the environment variable.
+  ///
+  /// - Returns: Whether or not the environment variable was successfully set.
+  @discardableResult
+  static func setVariable(_ value: String?, named name: String) -> Bool {
+#if SWT_NO_ENVIRONMENT_VARIABLES
+    simulatedEnvironment.withLock { environment in
+      environment[name] = value
+    }
+    return true
+#elseif SWT_TARGET_OS_APPLE || os(Linux) || os(FreeBSD) || os(OpenBSD) || os(Android) || os(WASI)
+    if let value {
+      return 0 == setenv(name, value, 1)
+    }
+    return 0 == unsetenv(name)
+#elseif os(Windows)
+    name.withCString(encodedAs: UTF16.self) { name in
+      if let value {
+        return value.withCString(encodedAs: UTF16.self) { value in
+          SetEnvironmentVariableW(name, value)
+        }
+      }
+      return SetEnvironmentVariableW(name, nil)
+    }
+#else
+#warning("Platform-specific implementation missing: environment variables unavailable")
+    return false
+#endif
+  }
+}

Tests/TestingTests/Support/EnvironmentTests.swift

@@ -71,42 +71,3 @@ struct EnvironmentTests {
     #expect(Environment.flag(named: name) == false)
   }
 }
-
-// MARK: - Fixtures
-
-extension Environment {
-  /// Set the environment variable with the specified name.
-  ///
-  /// - Parameters:
-  ///   - value: The new value for the specified environment variable. Pass
-  ///     `nil` to remove the variable from the current process' environment.
-  ///   - name: The name of the environment variable.
-  ///
-  /// - Returns: Whether or not the environment variable was successfully set.
-  @discardableResult
-  static func setVariable(_ value: String?, named name: String) -> Bool {
-#if SWT_NO_ENVIRONMENT_VARIABLES
-    simulatedEnvironment.withLock { environment in
-      environment[name] = value
-    }
-    return true
-#elseif SWT_TARGET_OS_APPLE || os(Linux) || os(FreeBSD) || os(OpenBSD) || os(Android) || os(WASI)
-    if let value {
-      return 0 == setenv(name, value, 1)
-    }
-    return 0 == unsetenv(name)
-#elseif os(Windows)
-    name.withCString(encodedAs: UTF16.self) { name in
-      if let value {
-        return value.withCString(encodedAs: UTF16.self) { value in
-          SetEnvironmentVariableW(name, value)
-        }
-      }
-      return SetEnvironmentVariableW(name, nil)
-    }
-#else
-#warning("Platform-specific implementation missing: environment variables unavailable")
-    return false
-#endif
-  }
-}