build b3sum binaries in CI for new tags

These configs and code are adapted from the CI workflow in
https://github.com/oconnor663/blake3-py, especially the
upload_github_release_asset.py script, which is copied verbatim.

.github/workflows/build_b3sum.py

@@ -0,0 +1,37 @@
+#! /usr/bin/env python3
+
+from pathlib import Path
+import platform
+import shutil
+import subprocess
+import sys
+
+ROOT = Path(__file__).parent.parent.parent
+RUST_TARGET = sys.argv[1]
+
+subprocess.run(["cargo", "build", "--target", sys.argv[1], "--release"],
+               cwd=ROOT / "b3sum")
+
+if platform.system() == "Windows":
+    original_exe_name = "b3sum.exe"
+else:
+    original_exe_name = "b3sum"
+
+if platform.system() == "Windows":
+    new_exe_name = "b3sum_windows_x64_bin.exe"
+elif platform.system() == "Darwin":
+    new_exe_name = "b3sum_macos_x64_bin"
+elif platform.system() == "Linux":
+    new_exe_name = "b3sum_linux_x64_bin"
+else:
+    raise RuntimeError("Unexpected platform: " + platform.system())
+
+# Copy the built binary so that it has the upload name we want.
+out_dir = ROOT / "b3sum/target" / RUST_TARGET / "release"
+original_exe_path = str(out_dir / original_exe_name)
+new_exe_path = str(out_dir / new_exe_name)
+print("copying", repr(original_exe_path), "to", repr(new_exe_path))
+shutil.copyfile(original_exe_path, new_exe_path)
+
+# This lets the subsequent upload step get the filepath.
+print("::set-output name=bin_path::" + new_exe_path)

.github/workflows/tag.yml

@@ -0,0 +1,45 @@
+name: publish_b3sum_binaries
+
+on:
+  push:
+    tags:
+      - "*"
+
+env:
+  BLAKE3_CI: "1"
+  RUSTFLAGS: "-D warnings"
+
+jobs:
+  cargo_tests:
+    name: ${{ matrix.target.name }}
+    runs-on: ${{ matrix.target.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        target: [
+          { "os": "ubuntu-latest",  "rust-target": "x86_64-unknown-linux-musl", "name": "Linux" },
+          { "os": "macOS-latest",   "rust-target": "x86_64-apple-darwin",       "name": "macOS" },
+          { "os": "windows-latest", "rust-target": "x86_64-pc-windows-msvc",    "name": "Windows" },
+        ]
+
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions/setup-python@v1
+        with:
+          python-version: "3.x"
+      - run: pip install PyGithub
+      - run: sudo apt-get install musl-tools
+        if: matrix.target.os == 'ubuntu-latest'
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          profile: minimal
+      - run: rustup target add ${{ matrix.target.rust-target }}
+      - name: build b3sum
+        id: build_b3sum
+        run: python -u .github/workflows/build_b3sum.py ${{ matrix.target.rust-target }}
+      - name: upload release asset
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TAG: ${{ github.ref }}
+        run: python -u .github/workflows/upload_github_release_asset.py ${{ steps.build_b3sum.outputs.bin_path }}

.github/workflows/upload_github_release_asset.py

@@ -0,0 +1,65 @@
+#! /usr/bin/env python3
+
+import github
+import os
+import sys
+
+RETRIES = 10
+
+g = github.Github(os.environ["GITHUB_TOKEN"])
+tag_name = os.environ["GITHUB_TAG"]
+tag_prefix = "refs/tags/"
+if tag_name.startswith(tag_prefix):
+    tag_name = tag_name[len(tag_prefix):]
+assert len(sys.argv) == 2
+asset_path = sys.argv[1]
+asset_name = os.path.basename(asset_path)
+
+repo = g.get_repo(os.environ["GITHUB_REPOSITORY"])
+
+tags = list(repo.get_tags())
+
+for tag in tags:
+    if tag.name == tag_name:
+        break
+else:
+    raise RuntimeError("no tag named " + repr(tag_name))
+
+try:
+    print("Creating GitHub release for tag " + repr(tag_name) + "...")
+    repo.create_git_release(tag_name, tag_name, tag.commit.commit.message)
+except github.GithubException as github_error:
+    if github_error.data["errors"][0]["code"] == "already_exists":
+        print("Release for tag " + repr(tag_name) + " already exists.")
+    else:
+        raise
+
+releases = list(repo.get_releases())
+for release in releases:
+    if release.tag_name == tag_name:
+        break
+else:
+    raise RuntimeError("no release for tag " + repr(tag_name))
+
+print("Uploading " + repr(asset_path) + "...")
+for i in range(RETRIES):
+    try:
+        print("Upload attempt #{} of {}...".format(i + 1, RETRIES))
+        release.upload_asset(asset_path)
+        break
+    except github.GithubException as github_error:
+        # Unfortunately the asset upload API is flaky. Even worse, it often
+        # partially succeeds, returning an error to the caller but leaving the
+        # release in a state where subsequent uploads of the same asset will
+        # fail with an "already_exists" error. (Though the asset is not visible
+        # on github.com, so we can't just declare victory and move on.) If we
+        # detect this case, explicitly delete the asset and continue retrying.
+        print(github_error)
+        for asset in release.get_assets():
+            if asset.name == asset_name:
+                print("Found uploaded asset after failure. Deleting...")
+                asset.delete_asset()
+else:
+    raise RuntimeError("All upload attempts failed.")
+
+print("Success!")

README.md

@@ -76,11 +76,14 @@ we recommend [Argon2](https://github.com/P-H-C/phc-winner-argon2).*
 
 ### The `b3sum` utility
 
-The `b3sum` utility allows you to process files and data from standard
-input using BLAKE3 in any of its three modes.
-To use `b3sum` on the command line, [install Rust and
+The `b3sum` command line utility prints the BLAKE3 hashes of files or of
+standard input. Prebuilt binaries are available for Linux, Windows, and
+macOS (requiring the [unidentified developer
+workaround](https://support.apple.com/guide/mac-help/open-a-mac-app-from-an-unidentified-developer-mh40616/mac))
+on the [releases page](https://github.com/BLAKE3-team/BLAKE3/releases).
+If you've [installed Rust and
 Cargo](https://doc.rust-lang.org/cargo/getting-started/installation.html),
-and then run:
+you can also build `b3sum` yourself with:
 
 ```bash
 cargo install b3sum
@@ -89,7 +92,7 @@ cargo install b3sum
 If `rustup` didn't configure your `PATH` for you, you might need to go
 looking for the installed binary in e.g. `~/.cargo/bin`. You can test
 out how fast BLAKE3 is on your machine by creating a big file and
-hashing it, for example as follows:
+hashing it, for example:
 
 ```bash
 # Create a 1 GB file.