add release documentation to CONTRIBUTING.md (#1171)

## Motivation

Currently, the process for releasing a new version of a Tokio crate is
somewhat complex, and is not well-documented. To make it easier for
contributors to release minor versions more frequently, there should be
documentation describing this process.

## Solution

This branch adds a section to `CONTRIBUTING.md` describing how to
release a new version of a Tokio crate. The steps are based on those
described by @carllerche in an offline conversation.

I've also added a quick shell script to actually publish new crate 
versions. This should make it harder to make mistakes when 
publishing.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

2 files changed, 177 insertions, 0 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index da207a2b..8bd28043 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -381,3 +381,63 @@ _Adapted from the [Node.js contributing guide][node]_.
 [node]: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 [hiding-a-comment]: https://help.github.com/articles/managing-disruptive-comments/#hiding-a-comment
 [documentation test]: https://doc.rust-lang.org/rustdoc/documentation-tests.html
+
+## Releasing
+
+Since the Tokio project consists of a number of crates, many of which depend on
+each other, releasing new versions to crates.io can involve some complexities.
+When releasing a new version of a crate, follow these steps:
+
+1. **Ensure that the release crate has no path dependencies.** When the HEAD
+   version of a Tokio crate requires unreleased changes in another Tokio crate,
+   the crates.io dependency on the second crate will be replaced with a path
+   dependency. Crates with path dependencies cannot be published, so before
+   publishing the dependent crate, any path dependencies must also be published.
+   This should be done through a form of depth-first tree traversal:
+
+   1. Starting with the first path dependency in the crate to be released,
+      inspect the `Cargo.toml` for the dependency. If the dependency has any
+      path dependencies of its own, repeat this step with the first such
+      dependency.
+   2. Begin the release process for the path dependency.
+   3. Once the path dependency has been published to crates.io, update the
+      dependent crate to depend on the crates.io version.
+   4. When all path dependencies have been published, the dependent crate may
+      be published.
+
+   To verify that a crate is ready to publish, run:
+
+   ```bash
+   bin/publish --dry-run <CRATE NAME> <CRATE VERSION>
+   ```
+
+2. **Update Cargo metadata.** After releasing any path dependencies, update the
+   `version` field in `Cargo.toml` to the new version, and the `documentation`
+   field to the docs.rs URL of the new version.
+3. **Update other documentation links.** Update the `#![doc(html_root_url)]`
+   attribute in the crate's `lib.rs` and the "Documentation" link in the crate's
+   `README.md` to point to the docs.rs URL of the new version.
+4. **Update the changelog for the crate.** Each crate in the Tokio repository
+   has its own `CHANGELOG.md` in that crate's subdirectory. Any changes to that
+   crate since the last release should be added to the changelog. Change
+   descriptions may be taken from the Git history, but should be edited to
+   ensure a consistent format, based on [Keep A Changelog][keep-a-changelog].
+   Other entries in that crate's changelog may also be used for reference.
+5. **Perform a final audit for breaking changes.** Compare the HEAD version of
+   crate with the Git tag for the most recent release version. If there are any
+   breaking API changes, determine if those changes can be made without breaking
+   existing APIs. If so, resolve those issues. Otherwise, if it is necessary to
+   make a breaking release, update the version numbers to reflect this.
+6. **Open a pull request with your changes.** Once that pull request has been
+   approved by a maintainer and the pull request has been merged, continue to
+   the next step.
+7. **Release the crate.** Run the following command:
+
+   ```bash
+   bin/publish <NAME OF CRATE> <VERSION>
+   ```
+
+   Your editor and prompt you to edit a message for the tag. Copy the changelog
+   entry for that release version into your editor and close the window.
+
+[keep-a-changelog]: https://github.com/olivierlacan/keep-a-changelog/blob/master/CHANGELOG.md
diff --git a/bin/publish b/bin/publish
new file mode 100755
index 00000000..df0669f4
--- /dev/null
+++ b/bin/publish
@@ -0,0 +1,117 @@
+#!/usr/bin/env bash
+set -e
+USAGE="Publish a new release of a tokio crate
+
+USAGE:
+    $(basename "$0") [OPTIONS] [CRATE] [VERSION]
+
+OPTIONS:
+    -v, --verbose   Use verbose Cargo output
+    -d, --dry-run   Perform a dry run (do not publish or tag the release)
+    -h, --help      Show this help text and exit"
+
+DRY_RUN=""
+VERBOSE=""
+
+err() {
+    echo -e "\e[31m\e[1merror:\e[0m $@" 1>&2;
+}
+
+status() {
+    WIDTH=12
+    printf "\e[32m\e[1m%${WIDTH}s\e[0m %s\n" "$1" "$2"
+}
+
+verify() {
+    status "Verifying" "if $CRATE v$VERSION can be released"
+    ACTUAL=$(cargo pkgid | sed -n 's/.*#\(.*\)/\1/p')
+
+    if [ "$ACTUAL" != "$VERSION" ]; then
+        err "expected to release version $VERSION, but Cargo.toml contained $ACTUAL"
+        exit 1
+    fi
+
+    if git tag -l | grep -Fxq "$TAG" ; then
+        err "git tag \`$TAG\` already exists"
+        exit 1
+    fi
+
+    PATH_DEPS=$(grep -F "path = \"" Cargo.toml | sed -e 's/^/  /')
+    if [ -n "$PATH_DEPS" ]; then
+        err "crate \`$CRATE\` contained path dependencies:\n$PATH_DEPS"
+        echo "path dependencies must be removed prior to release"
+        exit 1
+    fi
+}
+
+release() {
+    status "Releasing" "$CRATE v$VERSION"
+    cargo package $VERBOSE
+    cargo publish $VERBOSE $DRY_RUN
+
+    status "Tagging" "$TAG"
+    if [ -n "$DRY_RUN" ]; then
+        echo "# git tag $TAG && git push --tags"
+    else
+        git tag "$TAG" && git push --tags
+    fi
+}
+
+while [[ $# -gt 0 ]]
+do
+
+case "$1" in
+    -v|--verbose)
+    VERBOSE="--verbose"
+    set +x
+    shift
+    ;;
+    -d|--dry-run)
+    DRY_RUN="--dry-run"
+    shift
+    ;;
+    -*)
+    err "unknown flag \"$1\""
+    echo "$USAGE"
+    exit 1
+    ;;
+    *) # crate or version
+    if [ -z "$CRATE" ]; then
+        CRATE="$1"
+    elif [ -z "$VERSION" ]; then
+        VERSION="$1"
+    else
+        err "unknown positional argument \"$1\""
+        echo "$USAGE"
+        exit 1
+    fi
+    shift
+    ;;
+esac
+done
+# set -- "${POSITIONAL[@]}"
+
+if [ -z "$VERSION" ]; then
+    err "no version specified!"
+    HELP=1
+fi
+
+if [ -n "$CRATE" ]; then
+    TAG="$CRATE-$VERSION"
+else
+    err "no crate specified!"
+    HELP=1
+fi
+
+if [ -n "$HELP" ]; then
+    echo "$USAGE"
+    exit 1
+fi
+
+if [ -d "$CRATE" ]; then
+    (cd "$CRATE" && verify && release )
+else
+    err "no such crate \"$CRATE\""
+    exit 1
+fi
+