Generate cache key based on Job invocation

Attempt to capture as much context as possible about the job run
to generate a unique cache key. Unfortunately much of the matrix context
is not available to the action implementation.

src/cache-utils.ts

@@ -1,5 +1,6 @@
 import * as core from '@actions/core'
 import * as github from '@actions/github'
+import * as crypto from 'crypto'
 
 export function isCacheReadEnabled(cacheName: string): boolean {
     const configValue = getCacheEnabledValue(cacheName)
@@ -25,19 +26,48 @@ function getCacheEnabledValue(cacheName: string): string {
 }
 
 export function generateCacheKey(cacheName: string): CacheKey {
-    const cacheKeySeed = process.env[`CACHE_KEY_SEED`] || ''
-    const runnerOs = process.env[`RUNNER_OS`] || ''
-    const cacheKeyPrefix = `${cacheKeySeed}${runnerOs}|${cacheName}|`
+    // Prefix can be used to force change all cache keys
+    const cacheKeyPrefix = process.env['CACHE_KEY_PREFIX'] || ''
 
-    const args = truncateArgs(core.getInput('arguments'))
-    const cacheKeyWithArgs = `${cacheKeyPrefix}${args}|`
+    // At the most general level, share caches for all executions on the same OS
+    const runnerOs = process.env['RUNNER_OS'] || ''
+    const cacheKeyForOs = `${cacheKeyPrefix}${cacheName}|${runnerOs}`
 
-    const cacheKey = `${cacheKeyWithArgs}${github.context.sha}`
-    return new CacheKey(cacheKey, [cacheKeyWithArgs, cacheKeyPrefix])
+    // Prefer caches that run this job
+    const cacheKeyForJob = `${cacheKeyForOs}|${github.context.job}`
+
+    // Prefer (even more) jobs that run this job with the same context (matrix)
+    const cacheKeyForJobContext = `${cacheKeyForJob}[${determineJobContext()}]`
+
+    // Exact match on Git SHA
+    const cacheKey = `${cacheKeyForJobContext}-${github.context.sha}`
+
+    return new CacheKey(cacheKey, [
+        cacheKeyForJobContext,
+        cacheKeyForJob,
+        cacheKeyForOs
+    ])
 }
 
-function truncateArgs(args: string): string {
-    return args.trim().replace(/\s+/g, ' ').substr(0, 400)
+function determineJobContext(): string {
+    // Ideally we'd serialize the entire matrix values here, but matrix is not available within the action invocation.
+    // Use the JAVA_HOME value as a proxy for the java version
+    const javaHome = process.env['JAVA_HOME'] || ''
+
+    // Approximate overall context based on the first gradle invocation in the Job
+    const args = core.getInput('arguments')
+    const buildRootDirectory = core.getInput('build-root-directory')
+    const gradleVersion = core.getInput('gradle-version')
+
+    return hashStrings([javaHome, args, buildRootDirectory, gradleVersion])
+}
+
+export function hashStrings(values: string[]): string {
+    const hash = crypto.createHash('md5')
+    for (const value of values) {
+        hash.update(value)
+    }
+    return hash.digest('hex')
 }
 
 export class CacheKey {