cue/stats: start reporting which CUE evaluator version was used

Currently, when we select an evaluator version via CUE_EXPERIMENT
or via the Go API, it can be hard to really tell whether that version
is actually being used as part of evaluation. It may not be due to bugs,
for example, or because the way we are trying to select the version
doesn't actually work as intended, such as CUE_EXPERIMENT not affecting
the Go API until https://cuelang.org/cl/1202685.

As agreed with Marcel, a nice way for the evaluator to report the
version it used is the same way it already knows how to report other
information back to the user about the evaluation: CUE_STATS_FILE.

Note that it's currently not possible to collect these stats via the
exposed Go API, and that is a separate problem that we need to tackle.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I6c868b44dde88ca491570c526d1e2f2f0503f1d1
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202746
Reviewed-by: Marcel van Lohuizen <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

cmd/cue/cmd/testdata/script/stats.txtar

@@ -22,6 +22,15 @@ env CUE_STATS_FILE=-
 exec cue eval x.cue
 cmp stderr out/stderr
 
+# Check that EvalVersion reports the correct value.
+env CUE_STATS_FILE=-
+env CUE_EXPERIMENT=evalv3=0
+exec cue eval x.cue
+stderr -count=1 '"EvalVersion": 2,'
+env CUE_EXPERIMENT=evalv3=1
+exec cue eval x.cue
+stderr -count=1 '"EvalVersion": 3,'
+
 -- x.cue --
 a: 1
 b: 2
@@ -36,6 +45,7 @@ contents overwritten
 -- out/stats.json --
 {
     "CUE": {
+        "EvalVersion": 2,
         "Unifications": 4,
         "Disjuncts": 6,
         "Conjuncts": 8,
@@ -51,6 +61,7 @@ contents overwritten
 }
 -- out/stats.cue --
 CUE: {
+	EvalVersion:  2
 	Unifications: 4
 	Disjuncts:    6
 	Conjuncts:    8
@@ -65,6 +76,7 @@ Go: {
 }
 -- out/stats.yaml --
 CUE:
+  EvalVersion: 2
   Unifications: 4
   Disjuncts: 6
   Conjuncts: 8
@@ -78,6 +90,7 @@ Go:
 -- out/stderr --
 {
     "CUE": {
+        "EvalVersion": 2,
         "Unifications": 4,
         "Disjuncts": 6,
         "Conjuncts": 8,

cue/stats/stats.go

@@ -20,12 +20,23 @@ import (
 	"strings"
 	"sync"
 	"text/template"
+
+	"cuelang.org/go/internal"
 )
 
 // Counts holds counters for key events during a CUE evaluation.
 //
 // This is an experimental type and the contents may change without notice.
 type Counts struct {
+	// Note that we can't use the public [cuecontext.EvalVersion] type
+	// as that would lead to an import cycle. We could use "int" but that's a bit odd.
+	// There's no harm in referencing an internal type in practice, given that
+	// the public type is a type alias for the internal type already.
+
+	// EvalVersion is the evaluator version which was used for the CUE evaluation,
+	// corresponding to one of the values under [cuelang.org/go/cue/cuecontext.EvalVersion].
+	EvalVersion internal.EvaluatorVersion
+
 	// Operation counters
 	//
 	// These counters account for several key operations.
@@ -69,6 +80,22 @@ type Counts struct {
 // add checks on each of the operations.
 
 func (c *Counts) Add(other Counts) {
+	switch v, vo := c.EvalVersion, other.EvalVersion; {
+	case v == internal.EvalVersionUnset:
+		// The first time we add evaluator counts, we record the evaluator version being used.
+		if vo == internal.EvalVersionUnset {
+			panic("the first call to Counts.Add must provide an evaluator version")
+		}
+		c.EvalVersion = vo
+	case v != vo:
+		// Any further evaluator counts being added must match the same evaluator version.
+		//
+		// TODO(mvdan): this is currently not possible to enforce, as we collect stats globally
+		// via [adt.AddStats] which includes stats from contexts created with different versions.
+		// We likely need to refactor the collection of stats so that it is not global first.
+
+		// panic(fmt.Sprintf("cannot mix evaluator versions in Counts.Add: %v vs %v", v, vo))
+	}
 	c.Unifications += other.Unifications
 	c.Conjuncts += other.Conjuncts
 	c.Disjuncts += other.Disjuncts

internal/core/adt/context.go

@@ -186,6 +186,7 @@ func New(v *Vertex, cfg *Config) *OpContext {
 		Format:      cfg.Format,
 		vertex:      v,
 		Version:     version,
+		stats:       stats.Counts{EvalVersion: version},
 		Config:      flags,
 		taskContext: schedConfig,
 	}