Parallelize & Partition Verification (rust-lang#229)

Shard Kani verification workflow across multiple runners by:

1. Running `kani list --format json`, which outputs a JSON file like
this:

```json
{
  "kani-version": "0.56.0",
  "file-version": "0.1",
  "standard-harnesses": {
    "src/lib.rs": [
      "proof3",
      "verify::proof2"
    ],
    "src/test.rs": [
      "test::proof4",
      "test::proof5"
    ]
  },
  "contract-harnesses": {
    "src/lib.rs": [
      "proof"
    ]
  },
  "contracts": [
    {
      "function": "bar",
      "file": "src/lib.rs",
      "harnesses": [
        "proof"
      ]
    }
  ],
  "totals": {
    "standard-harnesses": 4,
    "contract-harnesses": 1,
    "functions-under-contract": 1
  }
}
```
2. Extracting the harnesses inside `"standard-harnesses"` and
`"contract-harnesses"` into an array called `ALL_HARNESSES` and the
length of that array into `HARNESS_COUNT`. (So in this example,
`ALL_HARNESSES = [proof3, verify::proof2, test::proof4, test::proof5,
proof]` and `HARNESS_COUNT=5`).
3. Dividing the harnesses evenly between four workers. For example, if
worker 1's harnesses are `proof3` and `verify::proof2`, then we call
`kani verify-std --harness proof3 --harness verify::proof2 --exact`. The
`--exact` makes Kani look for the exact harness name. This is important
so that we don't match on partial patterns, e.g., if there is a harness
called "foo" and a harness called "foo_bar", passing `--harness foo`
without `--exact` would match against both harnesses, and then `foo_bar`
would run twice.
4. Also parallelize verification within a single runner by passing `-j`
to Kani.

I chose four workers somewhat arbitrarily--it makes each worker take
about 45 minutes to an hour to finish. I thought it was good to have a
balance between too few workers (which still makes us wait a while) and
too many workers (which makes you look through more logs to find where a
given harness is being verified). But happy to play with this number if
people have opinions.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

.github/workflows/kani.yml

@@ -17,25 +17,39 @@ defaults:
 
 jobs:
   check-kani-on-std:
-    name: Verify std library
+    name: Verify std library (partition ${{ matrix.partition }})
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
+        partition: [1, 2, 3, 4]
         include:
           - os: ubuntu-latest
             base: ubuntu
           - os: macos-latest
             base: macos
+      fail-fast: false
+
+    env:
+      # Define the index of this particular worker [1-WORKER_TOTAL]
+      WORKER_INDEX: ${{ matrix.partition }}
+      # Total number of workers running this step
+      WORKER_TOTAL: 4
+
     steps:
       # Step 1: Check out the repository
       - name: Checkout Repository
         uses: actions/checkout@v4
         with:
           path: head
           submodules: true
-
-      # Step 2: Run Kani on the std library (default configuration)
+
+      # Step 2: Install jq
+      - name: Install jq
+        if: matrix.os == 'ubuntu-latest'
+        run: sudo apt-get install -y jq
+
+      # Step 3: Run Kani on the std library (default configuration)
       - name: Run Kani Verification
         run: head/scripts/run-kani.sh --path ${{github.workspace}}/head
 

scripts/run-kani.sh

@@ -77,6 +77,22 @@ TOML_FILE=${KANI_TOML_FILE:-$DEFAULT_TOML_FILE}
 REPO_URL=${KANI_REPO_URL:-$DEFAULT_REPO_URL}
 BRANCH_NAME=${KANI_BRANCH_NAME:-$DEFAULT_BRANCH_NAME}
 
+# Unstable arguments to pass to Kani
+unstable_args="-Z function-contracts -Z mem-predicates -Z float-lib -Z c-ffi -Z loop-contracts"
+
+# Variables used for parallel harness verification
+# When we say "parallel," we mean two dimensions of parallelization:
+#   1. Sharding verification across multiple workers. The Kani workflow that calls this script defines WORKER_INDEX and WORKER_TOTAL for this purpose: 
+#   we shard verification across WORKER_TOTAL workers, where each worker has a unique WORKER_INDEX that it uses to derive its share of ALL_HARNESSES to verify.
+#   2. Within a single worker, we parallelize verification between multiple cores by invoking kani with -j.
+
+# Array of all of the harnesses in the repository, set in get_harnesses()
+declare -a ALL_HARNESSES
+# Length of ALL_HARNESSES, set in get_harnesses()
+declare -i HARNESS_COUNT
+# `kani list` JSON FILE_VERSION that the parallel verification command expects
+EXPECTED_JSON_FILE_VERSION="0.1"
+
 # Function to read commit ID from TOML file
 read_commit_from_toml() {
     local file="$1"
@@ -151,10 +167,50 @@ get_kani_path() {
     echo "$(realpath "$build_dir/scripts/kani")"
 }
 
-run_kani_command() {
+# Run kani list with JSON format and process with jq to extract harness names and total number of harnesses.
+# Note: The code to extract ALL_HARNESSES is dependent on `kani list --format json` FILE_VERSION 0.1.
+# (The FILE_VERSION variable is defined in Kani in the list module's output code, current path kani-driver/src/list/output.rs)
+# If FILE_VERSION changes, first update the ALL_HARNESSES extraction logic to work with the new format, if necessary,
+# then update EXPECTED_JSON_FILE_VERSION.
+get_harnesses() {
     local kani_path="$1"
-    shift
-    "$kani_path" "$@"
+    "$kani_path" list -Z list $unstable_args ./library --std --format json
+    local json_file_version=$(jq -r '.["file-version"]' "$WORK_DIR/kani-list.json")
+    if [[ $json_file_version != $EXPECTED_JSON_FILE_VERSION ]]; then
+        echo "Error: The JSON file-version in kani-list.json does not equal $EXPECTED_JSON_FILE_VERSION"
+        exit 1
+    fi
+    # Extract the harnesses inside "standard-harnesses" and "contract-harnesses" 
+    # into an array called ALL_HARNESSES and the length of that array into HARNESS_COUNT
+    ALL_HARNESSES=($(jq -r '
+        ([.["standard-harnesses"] | to_entries | .[] | .value[]] + 
+            [.["contract-harnesses"] | to_entries | .[] | .value[]]) | 
+        .[]
+    ' $WORK_DIR/kani-list.json))
+    HARNESS_COUNT=${#ALL_HARNESSES[@]}
+}
+
+# Given an array of harness names, run verification for those harnesses
+run_verification_subset() {
+    local kani_path="$1"
+    local harnesses=("${@:2}")  # All arguments after kani_path are harness names
+
+    # Build the --harness arguments
+    local harness_args=""
+    for harness in "${harnesses[@]}"; do
+        harness_args="$harness_args --harness $harness"
+    done
+
+    echo "Running verification for harnesses:"
+    printf '%s\n' "${harnesses[@]}"
+    "$kani_path" verify-std -Z unstable-options ./library \
+        $unstable_args \
+        $harness_args --exact \
+        -j \
+        --output-format=terse \
+        $command_args \
+        --enable-unstable \
+        --cbmc-args --object-bits 12
 }
 
 # Check if binary exists and is up to date
@@ -176,7 +232,6 @@ check_binary_exists() {
     return 1
 }
 
-
 main() {
     local build_dir="$WORK_DIR/kani_build"
 
@@ -209,19 +264,38 @@ main() {
     "$kani_path" --version
 
     if [[ "$run_command" == "verify-std" ]]; then
-        echo "Running Kani verify-std command..."
-        "$kani_path" verify-std -Z unstable-options ./library \
-            -Z function-contracts \
-            -Z mem-predicates \
-            -Z loop-contracts \
-            -Z float-lib \
-            -Z c-ffi \
-            $command_args \
-            --enable-unstable \
-            --cbmc-args --object-bits 12
+        if [[ -n "$WORKER_INDEX" && -n "$WORKER_TOTAL" ]]; then
+            echo "Running as parallel worker $WORKER_INDEX of $WORKER_TOTAL"
+            get_harnesses "$kani_path"
+
+            echo "All harnesses:"
+            printf '%s\n' "${ALL_HARNESSES[@]}"
+            echo "Total number of harnesses: $HARNESS_COUNT"
+
+            # Calculate this worker's portion (add WORKER_TOTAL - 1 to force ceiling division)
+            chunk_size=$(( (HARNESS_COUNT + WORKER_TOTAL - 1) / WORKER_TOTAL ))
+            echo "Number of harnesses this worker will run: $chunk_size"
+
+            start_idx=$(( (WORKER_INDEX - 1) * chunk_size ))
+            echo "Start index into ALL_HARNESSES is $start_idx"
+
+            # Extract this worker's harnesses
+            worker_harnesses=("${ALL_HARNESSES[@]:$start_idx:$chunk_size}")
+
+            # Run verification for this subset
+            run_verification_subset "$kani_path" "${worker_harnesses[@]}"
+        else
+            # Run verification for all harnesses (not in parallel)
+            echo "Running Kani verify-std command..."
+            "$kani_path" verify-std -Z unstable-options ./library \
+                $unstable_args
+                $command_args \
+                --enable-unstable \
+                --cbmc-args --object-bits 12
+        fi
     elif [[ "$run_command" == "list" ]]; then
         echo "Running Kani list command..."
-        "$kani_path" list -Z list -Z function-contracts -Z mem-predicates -Z float-lib -Z c-ffi ./library --std --format markdown
+        "$kani_path" list -Z list $unstable_args ./library --std --format markdown
     fi
 }