Add JDK installation support (Microsoft OpenJDK)

[rmarinho]

Summary

Adds JDK installation support to Xamarin.Android.Tools.AndroidSdk using the Microsoft Build of OpenJDK (https://www.microsoft.com/openjdk).

Closes #270

New Files

File	Purpose
JdkInstaller.cs	Discovers, downloads, installs, validates, and removes Microsoft OpenJDK
DownloadUtils.cs	Shared helpers: download with progress, SHA-256 checksum verification, Zip Slip-safe ZIP extraction, tar.gz extraction, checksum file fetching/parsing
Models/JdkVersionInfo.cs	Version metadata (major, minor, patch, build, platform, architecture, URL, checksum URL)
Models/JdkInstallPhase.cs	enum for progress reporting phases (Downloading, Verifying, Extracting, etc.)
Models/JdkInstallProgress.cs	record combining phase, percentage, and message for IProgress<T> callbacks
IsExternalInit.cs	Enables init accessors on netstandard2.0
.github/copilot-instructions.md	Repository-specific Copilot context

Modified Files

File	Changes
FileUtil.cs	Added TryDeleteFile, TryDeleteDirectory, MoveWithRollback, CommitMove, IsTargetPathWritable, IsUnderDirectory, GetInstallerExtension, GetArchiveExtension
ProcessUtils.cs	Added CreateProcessStartInfo (uses ArgumentList on net5+, JoinArguments fallback), IsElevated (uses Environment.IsPrivilegedProcess on net7+, P/Invoke fallback)
.csproj	Added System.Buffers 4.5.1 (netstandard2.0), IsExternalInit.cs exclusion on modern TFMs

API Surface

public class JdkInstaller : IDisposable
{
    public JdkInstaller(Action<string>? logger = null);
    public static int RecommendedMajorVersion { get; }          // 21
    public static IReadOnlyList<int> SupportedVersions { get; } // [21]
    public Task<IReadOnlyList<JdkVersionInfo>> DiscoverAsync(CancellationToken ct = default);
    public Task InstallAsync(int majorVersion, string targetPath,
        IProgress<JdkInstallProgress>? progress = null, CancellationToken ct = default);
    public bool IsValid(string jdkPath);
    public bool Remove(string jdkPath);
    public void Dispose();
}

// Helpers used by JdkInstaller (not on JdkInstaller itself):
// FileUtil.IsTargetPathWritable(string, Action<TraceLevel, string>)
// ProcessUtils.IsElevated()

Key Design Decisions

• Microsoft OpenJDK only — downloads from https://aka.ms/download-jdk with SHA-256 checksum verification
• JDK 21 only — RecommendedMajorVersion = 21, SupportedVersions = [21]. Array allows future additions
• Owns its HttpClient — no injection; Dispose() cleans it up
• Elevated install mode — when ProcessUtils.IsElevated() is true, uses platform-native silent installers (.msi via msiexec /qn, .pkg via /usr/sbin/installer). When not elevated, extracts archive to targetPath
• Safe directory replacement — MoveWithRollback: rename existing → temp, move new in place, delete temp via CommitMove. Avoids Delete/Move race on Windows
• Zip Slip protection — ZIP extraction validates that all entry paths resolve under the destination directory
• ArrayPool<byte> download buffer — ArrayPool<byte>.Shared.Rent for the 80 KB download buffer, returned in finally
• Cross-platform P/Invoke fallbacks — IsUserAnAdmin() (shell32) on Windows, geteuid() (libc) on Unix, behind #if !NET5_0_OR_GREATER
• NullProgress pattern — avoids null checks throughout install flow
• netstandard2.0 + net10.0 — conditional compilation for ArgumentList, Environment.IsPrivilegedProcess, ReadAsStringAsync/ReadAsStreamAsync overloads. Private helpers encapsulate #if blocks

Tests

66 unit tests across 5 test files (all target net10.0):

Test File	Count	Coverage
JdkInstallerTests.cs	19	IsValid, DiscoverAsync, InstallAsync, IsTargetPathWritable, Remove, properties
DownloadUtilsTests.cs	20	ParseChecksumFile (12 cases), VerifyChecksum (4), ExtractZipSafe + Zip Slip (4)
FileUtilTests.cs	15	MoveWithRollback (4), IsUnderDirectory (11 including cross-platform path cases)
ProcessUtilsTests.cs	7	CreateProcessStartInfo (6), IsElevated smoke (1)
JdkVersionInfoTests.cs	5	Constructor, defaults, ToString, mutable properties

Review Feedback Addressed

JDK 21 only (removed 17), removed HttpClient injection, IDisposable, CancellationToken pattern, path normalization, validation cleanup, ArgumentList for tar, absolute /usr/bin/tar, null comparison pattern, one type per file, record type, checksum errors throw, test TearDown dispose, CI skip, elevated install mode, Zip Slip protection, ArrayPool buffer, extracted constants (BufferSize, BytesPerMB, WhitespaceChars), private ReadAsStreamAsync/ReadAsStringAsync helpers, cross-platform test paths, comprehensive unit test coverage

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

src/Xamarin.Android.Tools.AndroidSdk/JdkInstaller.cs:445

• When totalBytes is 0 (because both ContentLength and expectedSize are 0), the progress message will show "0 MB / 0 MB" which is acceptable, but the percentage calculation is skipped due to the check on line 443. However, if totalBytes is very small (less than 1 MB), the division by (1024 * 1024) will result in 0 for both values in the message, which could be confusing. Consider using KB or bytes for small files, or formatting the size more intelligently (e.g., using a helper method that chooses appropriate units).

				if (progress != null && totalBytes > 0) {
					var pct = (double) totalRead / totalBytes * 100;
					progress.Report (new JdkInstallProgress (JdkInstallPhase.Downloading, pct, $"Downloaded {totalRead / (1024 * 1024)} MB / {totalBytes / (1024 * 1024)} MB"));

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Comments suppressed due to low confidence (2)

src/Xamarin.Android.Tools.AndroidSdk/JdkInstaller.cs:207

• InstallAsync treats an empty string targetPath the same as null and throws ArgumentNullException. For API clarity, consider throwing ArgumentException for empty/whitespace paths and reserving ArgumentNullException for null only (matching typical .NET conventions).

				logger (TraceLevel.Error, $"JDK installation failed: {ex.Message}");
				logger (TraceLevel.Verbose, ex.ToString ());
				throw;
			}

src/Xamarin.Android.Tools.AndroidSdk/JdkInstaller.cs:571

• Zip Slip validation uses StringComparison.OrdinalIgnoreCase unconditionally. On case-sensitive filesystems (Linux/macOS), a ZIP entry with an absolute path that differs only by case (e.g. "/TMP/..." vs root "/tmp/...") can incorrectly pass the StartsWith check, allowing extraction outside the destination directory. Use Ordinal on non-Windows (and ideally explicitly reject rooted entry.FullName) and keep OrdinalIgnoreCase only for Windows.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

[Copilot]

Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

[jonathanpeppers]

I'm squashing/merging, getting a nice commit message.