test: add pipe checker to test dir

The pipe checker is an attempt at replicating how one would go to write
commands on a CLI and piping them together, coupled with a `test`.

test/pipe_checker_test.go

@@ -0,0 +1,86 @@
+package test
+
+import "fmt"
+
+// Pipe is any command that allows piping two gadgets together
+//
+// There's always only one input and one output (stdout), mimicking essentially
+// how pipes work in the UNIX-world.
+type Pipe interface {
+	Process(input string) (string, error)
+}
+
+// CustomPipe allows providing a simple function for piping inputs together
+type CustomPipe func(string) (string, error)
+
+func (c CustomPipe) Process(input string) (string, error) {
+	return c(input)
+}
+
+// Tester is the end of a pipe for testing purposes.
+//
+// Once multiple commands have been piped together in a pipeline, we can
+// perform some checks on that input, and decide if a test is a success or a
+// failure.
+type Tester interface {
+	Check(input string) error
+}
+
+// CustomTester allows providing a function to check that the input is what we want
+type CustomTester func(string) error
+
+func (ct CustomTester) Check(input string) error {
+	return ct(input)
+}
+
+// PipeChecker is a kind of checker that essentially lets users write mini
+// gadgets that pipe inputs together, and compose those to end as a true/false
+// statement, which translates to an error.
+//
+// Unlike pipes in a real command-line context, since we're dealing with
+// finite gadgets to process data, we're sequentially running their Process
+// function, and any processor that ends in an error will make the pipeline
+// fail.
+//
+// Stream is provided so we know if we want to combine stdout/stderr for the
+// pipeline, or if we want only to focus on either.
+type PipeChecker struct {
+	name   string
+	stream Stream
+
+	pipers []Pipe
+	check  Tester
+}
+
+func (pc PipeChecker) Check(stdout, stderr string, _ error) error {
+	if len(pc.pipers) == 0 {
+		return fmt.Errorf("%s - empty pipeline", pc.Name())
+	}
+
+	var pipeStr string
+	switch pc.stream {
+	case OnlyStdout:
+		pipeStr = stdout
+	case OnlyStderr:
+		pipeStr = stderr
+	case BothStreams:
+		pipeStr = fmt.Sprintf("%s\n%s", stdout, stderr)
+	}
+
+	var err error
+	for i, pp := range pc.pipers {
+		pipeStr, err = pp.Process(pipeStr)
+		if err != nil {
+			return fmt.Errorf("pipeline failed during execution (%d): %s", i, err)
+		}
+	}
+	return pc.check.Check(pipeStr)
+}
+
+func (pc PipeChecker) Name() string {
+	rawName := "|>?"
+	if pc.name != "" {
+		rawName = fmt.Sprintf("%s - %s", rawName, pc.name)
+	}
+	return rawName
+}