getproviders: Consistent ordering of terms in VersionConstraintsString

An earlier commit made this remove duplicates, which set the precedent
that this function is trying to canonically represent the _meaning_ of
the version constraints rather than exactly how they were expressed in
the configuration.

Continuing in that vein, now we'll also apply a consistent (though perhaps
often rather arbitrary) ordering to the terms, so that it doesn't change
due to irrelevant details like declarations being written in a different
order in the configuration.

The ordering here is intended to be reasonably intuitive for simple cases,
but constraint strings with many different constraints are hard to
interpret no matter how we order them so the main goal is consistency,
so those watching how the constraints change over time (e.g. in logs of
Terraform output, or in the dependency log file) will see fewer noisy
changes that don't actually mean anything.

internal/getproviders/types.go

@@ -395,16 +395,36 @@ func VersionConstraintsString(spec VersionConstraints) string {
 	// lock files. Therefore the canonical forms produced here are a compatibility
 	// constraint for the dependency lock file parser.
 
-	// Keep track of selection specifications which have been seen, so that we
-	// don't repeat the same constraint multiple times.
-	rendered := make(map[constraints.SelectionSpec]struct{})
+	if len(spec) == 0 {
+		return ""
+	}
+
+	// VersionConstraints values are typically assembled by combining together
+	// the version constraints from many separate declarations throughout
+	// a configuration, across many modules. As a consequence, they typically
+	// contain duplicates and the terms inside are in no particular order.
+	// For our canonical representation we'll both deduplicate the items
+	// and sort them into a consistent order.
+	sels := make(map[constraints.SelectionSpec]struct{})
+	for _, sel := range spec {
+		sels[sel] = struct{}{}
+	}
+	selsOrder := make([]constraints.SelectionSpec, 0, len(sels))
+	for sel := range sels {
+		selsOrder = append(selsOrder, sel)
+	}
+	sort.Slice(selsOrder, func(i, j int) bool {
+		is, js := selsOrder[i], selsOrder[j]
+		boundaryCmp := versionSelectionBoundaryCompare(is.Boundary, js.Boundary)
+		if boundaryCmp == 0 {
+			// The operator is the decider, then.
+			return versionSelectionOperatorLess(is.Operator, js.Operator)
+		}
+		return boundaryCmp < 0
+	})
 
 	var b strings.Builder
-	for i, sel := range spec {
-		// If we've already rendered this selection spec, skip it.
-		if _, exists := rendered[sel]; exists {
-			continue
-		}
+	for i, sel := range selsOrder {
 		if i > 0 {
 			b.WriteString(", ")
 		}
@@ -459,9 +479,77 @@ func VersionConstraintsString(spec VersionConstraints) string {
 		if sel.Boundary.Metadata != "" {
 			b.WriteString("+" + boundary.Metadata)
 		}
-
-		// Mark this selection spec as rendered.
-		rendered[sel] = struct{}{}
 	}
 	return b.String()
 }
+
+// Our sort for selection operators is somewhat arbitrary and mainly motivated
+// by consistency rather than meaning, but this ordering does at least try
+// to make it so "simple" constraint sets will appear how a human might
+// typically write them, with the lower bounds first and the upper bounds
+// last. Weird mixtures of different sorts of constraints will likely seem
+// less intuitive, but they'd be unintuitive no matter the ordering.
+var versionSelectionsBoundaryPriority = map[constraints.SelectionOp]int{
+	// We skip zero here so that if we end up seeing an invalid
+	// operator (which the string function would render as "???")
+	// then it will have index zero and thus appear first.
+	constraints.OpGreaterThan:                 1,
+	constraints.OpGreaterThanOrEqual:          2,
+	constraints.OpEqual:                       3,
+	constraints.OpGreaterThanOrEqualPatchOnly: 4,
+	constraints.OpGreaterThanOrEqualMinorOnly: 5,
+	constraints.OpLessThanOrEqual:             6,
+	constraints.OpLessThan:                    7,
+	constraints.OpNotEqual:                    8,
+}
+
+func versionSelectionOperatorLess(i, j constraints.SelectionOp) bool {
+	iPrio := versionSelectionsBoundaryPriority[i]
+	jPrio := versionSelectionsBoundaryPriority[j]
+	return iPrio < jPrio
+}
+
+func versionSelectionBoundaryCompare(i, j constraints.VersionSpec) int {
+	// In the Ruby-style constraint syntax, unconstrained parts appear
+	// only for omitted portions of a version string, like writing
+	// "2" instead of "2.0.0". For sorting purposes we'll just
+	// consider those as zero, which also matches how we serialize them
+	// to strings.
+	i, j = i.ConstrainToZero(), j.ConstrainToZero()
+
+	// Once we've removed any unconstrained parts, we can safely
+	// convert to our main Version type so we can use its ordering.
+	iv := Version{
+		Major:      i.Major.Num,
+		Minor:      i.Minor.Num,
+		Patch:      i.Patch.Num,
+		Prerelease: versions.VersionExtra(i.Prerelease),
+		Metadata:   versions.VersionExtra(i.Metadata),
+	}
+	jv := Version{
+		Major:      j.Major.Num,
+		Minor:      j.Minor.Num,
+		Patch:      j.Patch.Num,
+		Prerelease: versions.VersionExtra(j.Prerelease),
+		Metadata:   versions.VersionExtra(j.Metadata),
+	}
+	if iv.Same(jv) {
+		// Although build metadata doesn't normally weigh in to
+		// precedence choices, we'll use it for our visual
+		// ordering just because we need to pick _some_ order.
+		switch {
+		case iv.Metadata.Raw() == jv.Metadata.Raw():
+			return 0
+		case iv.Metadata.LessThan(jv.Metadata):
+			return -1
+		default:
+			return 1 // greater, by elimination
+		}
+	}
+	switch {
+	case iv.LessThan(jv):
+		return -1
+	default:
+		return 1 // greater, by elimination
+	}
+}

internal/getproviders/types_test.go

@@ -43,7 +43,7 @@ func TestVersionConstraintsString(t *testing.T) {
 		},
 		"other operators": {
 			MustParseVersionConstraints("> 1.0.0, < 1.0.0, >= 1.0.0, <= 1.0.0, != 1.0.0"),
-			"> 1.0.0, < 1.0.0, >= 1.0.0, <= 1.0.0, != 1.0.0",
+			"> 1.0.0, >= 1.0.0, <= 1.0.0, < 1.0.0, != 1.0.0",
 		},
 		"multiple": {
 			MustParseVersionConstraints(">= 3.0, < 4.0"),
@@ -51,7 +51,33 @@ func TestVersionConstraintsString(t *testing.T) {
 		},
 		"duplicates removed": {
 			MustParseVersionConstraints(">= 1.2.3, 1.2.3, ~> 1.2, 1.2.3"),
-			">= 1.2.3, 1.2.3, ~> 1.2",
+			"~> 1.2, >= 1.2.3, 1.2.3",
+		},
+		"consistent ordering, exhaustive": {
+			// This weird jumble is just to exercise the different sort
+			// ordering codepaths. Hopefully nothing quite this horrific
+			// shows up often in practice.
+			MustParseVersionConstraints("< 1.2.3, <= 1.2.3, != 1.2.3, 1.2.3+local.2, 1.2.3+local.1, = 1.2.4, = 1.2.3, > 2, > 1.2.3, >= 1.2.3, ~> 1.2.3, ~> 1.2"),
+			"~> 1.2, > 1.2.3, >= 1.2.3, 1.2.3, ~> 1.2.3, <= 1.2.3, < 1.2.3, != 1.2.3, 1.2.3+local.1, 1.2.3+local.2, 1.2.4, > 2.0.0",
+		},
+		"consistent ordering, more typical": {
+			// This one is aiming to simulate a common situation where
+			// various different modules express compatible constraints
+			// but some modules are more constrained than others. The
+			// combined results here can be kinda confusing, but hopefully
+			// ordering them consistently makes them a _little_ easier to read.
+			MustParseVersionConstraints("~> 1.2, >= 1.2, 1.2.4"),
+			">= 1.2.0, ~> 1.2, 1.2.4",
+		},
+		"consistent ordering, disjoint": {
+			// One situation where our presentation of version constraints is
+			// particularly important is when a user accidentally ends up with
+			// disjoint constraints that can therefore never match. In that
+			// case, our ordering should hopefully make it easier to determine
+			// that the constraints are disjoint, as a first step to debugging,
+			// by showing > or >= constrains sorted after < or <= constraints.
+			MustParseVersionConstraints(">= 2, >= 1.2, < 1.3"),
+			">= 1.2.0, < 1.3.0, >= 2.0.0",
 		},
 	}
 	for name, tc := range testCases {