A pseudo code example of an approach to doing Bazel Target Diffing

BazelTargetDiffing.swift

func main() {
    // convertToBazelSourceTargets should take a filepath in your repo and return a bazel source target path
    // Use bazel query to do this
    let modifiedFileSourceTargets = convertToBazelSourceTargets(system("git diff --name-only HEAD^ HEAD"))
    let initialTargetShas = createTargetShas()
    system("git checkout HEAD^")
    let finalTargetShas = createTargetShas()
    system("git checkout -")
    var modifiedTargets: [String] = []
    for target, value in finalTargetShas {
        if initialTargetShas[target] != value || initialTargetShas[target] == nil {
            modifiedTargets.append(target)
        }
    }
    var targetsToTest: Set<String> = .init()
    for modifiedTarget in modifiedTargets {
        // queryRDepsForTarget: Run a bazel rdeps query on the target and return an array of targets
        targetsToTest = targetsToTest.union(queryRDepsForTarget(modifiedTarget))
    }
    let query = targetsToTest.joined(" ")
    system("bazel test \(query)")
}

private func createTargetShas(modifiedFileSourceTargets: [String]) -> [String: String] {
    let protoFileName = UUID() + ".proto"
    let queryCommand = "bazel query \"'//external:all-targets' + '//...:all-targets'\" --output proto > \(protoFileName)"
    system(queryCommand)
    // You will need Proto models for this, find them here https://github.com/bazelbuild/bazel/blob/master/src/main/protobuf/build.proto
    let bazelQuery = BlazeQuery_Rule(serializedData: Data(protoFileName))
    var targetShas[String: String] = [:]
    let targets = bazelQuery.targets.filter{ $0.hasRule }
    for target in targets {
        targetShas[target.rule.name] = createShaForRule(target.rule,
                                                        modifiedFileSourceTargets: modifiedFileSourceTargets).hexString
    }
    defer { system("rm \(protoFileName)") }
    return targetShas
}

private func createShaForRule(rule: Blaze_Rule,
                              modifiedFileSourceTargets: [String]) -> SHA256.Digest {
    var sha: SHA256 = .init()
    sha.update(rule.ruleClass.data(using: .utf8))
    sha.update(rule.name.data(using: .utf8))
    sha.update(rule.skylarkEnvironmentHashCode.data(using: .utf8))
    for attribute in rule.attribute {
        sha.update(attribute.serializedData)
    }
    for ruleInput in rule.ruleInputs {
        // Use hashmaps to implement isRule
        if ruleInput.isRule {
            sha.update(createShaForRule(rule: ruleInput).asData)
        } else if let modifiedPath = modifiedFileSourceTargets.first?(ruleInput) {
            // RuleInputs always contain the set of source file targets needed for a rule, we will 
            // use this to cross reference the modified files from Git and read the data to create
            // Shas for the modified file targets
            sha.update(Data(modifiedPath))
        }
    }
    sha.finalize()
}

Thanks for posting this. I've only taken a quick look through the code (and don't know Swift), but I'm wondering if this supports the case where a file/target has been deleted and thus won't match to anything in a bazel query?

Thanks for posting this. I've only taken a quick look through the code (and don't know Swift), but I'm wondering if this supports the case where a file/target has been deleted and thus won't match to anything in a bazel query?

Yes this will work for files that are deleted or moved, I just tested it now