Fix swiftc selection for the case no swiftc found in Swift SDK (#7296)

Fix `PATH` executable selection in `UserToolchain`

### Motivation:

When `swiftc` is not in the toolchain bin directory of Swift SDK, system
swiftc in PATH should be used. But the current implementation of
`UserToolchain` always picks the last found entry in the PATH instead of
the first one. This behavior was introduced by
22c2493

The causes actual issues when:
1. More than two `swiftc` are in `PATH`
2. The invoked `swift-build` is placed from outside of toolchain bin
directory

Then it uses the last found `swiftc` in `PATH` even though we expect the
first one.

### Modifications:

Fixed the order of `PATH` selection to prefer the first entry.

### Result:

The first found `swiftc` is used as well as shell PATH selection.

Author: kateinoigakukun

Sources/PackageModel/UserToolchain.swift

@@ -128,6 +128,8 @@ public final class UserToolchain: Toolchain {
                 continue
             }
             toolPath = path
+            // Take the first match.
+            break
         }
         guard let toolPath else {
             throw InvalidToolchainDiagnostic("could not find CLI tool `\(name)` at any of these directories: \(binDirectories)")

Tests/PackageModelTests/PackageModelTests.swift

@@ -76,19 +76,20 @@ class PackageModelTests: XCTestCase {
         )
     }
 
+    // tiny PE binary from: https://archive.is/w01DO
+    static let tinyPEBytes: [UInt8] = [
+        0x4D, 0x5A, 0x00, 0x00, 0x50, 0x45, 0x00, 0x00, 0x4C, 0x01, 0x01, 0x00,
+        0x6A, 0x2A, 0x58, 0xC3, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x04, 0x00, 0x03, 0x01, 0x0B, 0x01, 0x08, 0x00, 0x04, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x0C, 0x00, 0x00, 0x00,
+        0x04, 0x00, 0x00, 0x00, 0x0C, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x00,
+        0x04, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x68, 0x00, 0x00, 0x00, 0x64, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x02,
+    ]
+
     func testWindowsLibrarianSelection() throws {
-        // tiny PE binary from: https://archive.is/w01DO
-        let contents: [UInt8] = [
-            0x4D, 0x5A, 0x00, 0x00, 0x50, 0x45, 0x00, 0x00, 0x4C, 0x01, 0x01, 0x00,
-            0x6A, 0x2A, 0x58, 0xC3, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-            0x04, 0x00, 0x03, 0x01, 0x0B, 0x01, 0x08, 0x00, 0x04, 0x00, 0x00, 0x00,
-            0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x0C, 0x00, 0x00, 0x00,
-            0x04, 0x00, 0x00, 0x00, 0x0C, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x00,
-            0x04, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00,
-            0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-            0x68, 0x00, 0x00, 0x00, 0x64, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-            0x02,
-        ]
 
         #if os(Windows)
         let suffix = ".exe"
@@ -99,8 +100,9 @@ class PackageModelTests: XCTestCase {
         let triple = try Triple("x86_64-unknown-windows-msvc")
         let fs = localFileSystem
 
-        try withTemporaryFile { [contents] _ in
-            try withTemporaryDirectory(removeTreeOnDeinit: true) { [contents] tmp in
+        try withTemporaryFile { _ in
+            try withTemporaryDirectory(removeTreeOnDeinit: true) { tmp in
+                let contents = Self.tinyPEBytes
                 let bin = tmp.appending("bin")
                 try fs.createDirectory(bin)
 
@@ -140,4 +142,40 @@ class PackageModelTests: XCTestCase {
             }
         }
     }
+
+    func testDetermineSwiftCompilers() throws {
+        let fs = localFileSystem
+        try withTemporaryFile { _ in
+            try withTemporaryDirectory(removeTreeOnDeinit: true) { tmp in
+                // When swiftc is not in the toolchain bin directory, UserToolchain
+                // should find it in the system PATH search paths in the order they
+                // are specified.
+                let toolchainPath = tmp.appending("swift.xctoolchain")
+                let toolchainBinDir = toolchainPath.appending(components: "usr", "bin")
+                // Create the toolchain bin directory, but don't put swiftc in it.
+                try fs.createDirectory(toolchainBinDir, recursive: true)
+
+                // Create a directory with two swiftc binaries in it.
+                let binDirs = ["bin1", "bin2"].map { tmp.appending($0) }
+                for binDir in binDirs {
+                    try fs.createDirectory(binDir)
+                    let binFile = binDir.appending("swiftc")
+                    try fs.writeFileContents(binFile, bytes: ByteString(Self.tinyPEBytes))
+                    #if !os(Windows)
+                    try fs.chmod(.executable, path: binFile, options: [])
+                    #endif
+                }
+
+                let compilers = try UserToolchain.determineSwiftCompilers(
+                    binDirectories: [toolchainBinDir],
+                    useXcrun: false,
+                    environment: [:],
+                    searchPaths: binDirs
+                )
+
+                // The first swiftc in the search paths should be chosen.
+                XCTAssertEqual(compilers.compile, binDirs.first?.appending("swiftc"))
+            }
+        }
+    }
 }