hclsyntax: Impose an upper limit on a refined prefix in TemplateExpr

There is no limit to the length of string prefixes produced by template
expressions, so in rare cases they may return a refined unknown string
has too long a prefix.

The cty's msgpack decoder limits the size of an acceptable refinements
to 1 kiB, so such a value cannot be handled and an error occurs.

This change limits the length of prefixes to 128 B, so overly long
prefixes are no longer an issue in most cases.

hclsyntax/expression_template.go

@@ -94,7 +94,16 @@ func (e *TemplateExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 		ret = cty.UnknownVal(cty.String)
 		if !diags.HasErrors() { // Invalid input means our partial result buffer is suspect
 			if knownPrefix := buf.String(); knownPrefix != "" {
-				ret = ret.Refine().StringPrefix(knownPrefix).NewValue()
+				byteLen := len(knownPrefix)
+				// Impose a reasonable upper limit to avoid producing too long a prefix.
+				// The 128 B is about 10% of the safety limits in cty's msgpack decoder.
+				// @see https://github.com/zclconf/go-cty/blob/v1.13.2/cty/msgpack/unknown.go#L170-L175
+				//
+				// This operation is safe because StringPrefix removes incomplete trailing grapheme clusters.
+				if byteLen > 128 { // arbitrarily-decided threshold
+					byteLen = 128
+				}
+				ret = ret.Refine().StringPrefix(knownPrefix[:byteLen]).NewValue()
 			}
 		}
 	} else {

hclsyntax/expression_template_test.go

@@ -4,6 +4,7 @@
 package hclsyntax
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/hcl/v2"
@@ -307,6 +308,16 @@ trim`,
 			cty.UnknownVal(cty.String).Refine().NotNull().StringPrefixFull("test_known_").NewValue(),
 			0,
 		},
+		{ // can preserve a static prefix as a refinement, but the length is limited to 128 B
+			strings.Repeat("_", 130) + `${unknown}`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"unknown": cty.UnknownVal(cty.String),
+				},
+			},
+			cty.UnknownVal(cty.String).Refine().NotNull().StringPrefixFull(strings.Repeat("_", 128)).NewValue(),
+			0,
+		},
 		{ // marks from uninterpolated values are ignored
 			`hello%{ if false } ${target}%{ endif }`,
 			&hcl.EvalContext{