add TF_LOG_TRACE to turn off graph trace output

Graph transformations are really only interesting for developers
directly working on graph transformers. Otherwise the complete graph can
be printed once in the trace log for general debugging purposes.
Removing the intermediate graph steps can reduce log output for large
configs by many megabytes per run. On top of the volume of output, the
graph string generation can cause a noticeable impact on performance with
large graphs.

internal/logging/logging.go

@@ -27,6 +27,10 @@ const (
 	envLogProvider = "TF_LOG_PROVIDER"
 	envLogCloud    = "TF_LOG_CLOUD"
 	envLogStacks   = "TF_LOG_STACKS"
+
+	// This variable is defined here for consistency, but it is used by the
+	// graph builder directly to change how much output to generate.
+	envGraphTrace = "TF_GRAPH_TRACE"
 )
 
 var (
@@ -44,6 +48,8 @@ var (
 		panics:   make(map[string][]string),
 		maxLines: 100,
 	}
+
+	GraphTrace = os.Getenv(envGraphTrace) != ""
 )
 
 func init() {

internal/terraform/graph_builder.go

@@ -45,11 +45,14 @@ func (b *BasicGraphBuilder) Build(path addrs.ModuleInstance) (*Graph, tfdiags.Di
 		log.Printf("[TRACE] Executing graph transform %T", step)
 
 		err := step.Transform(g)
-		if thisStepStr := g.StringWithNodeTypes(); thisStepStr != lastStepStr {
-			log.Printf("[TRACE] Completed graph transform %T with new graph:\n%s  ------", step, logging.Indent(thisStepStr))
-			lastStepStr = thisStepStr
-		} else {
-			log.Printf("[TRACE] Completed graph transform %T (no changes)", step)
+
+		if logging.GraphTrace {
+			if thisStepStr := g.StringWithNodeTypes(); thisStepStr != lastStepStr {
+				log.Printf("[TRACE] Completed graph transform %T with new graph:\n%s  ------", step, logging.Indent(thisStepStr))
+				lastStepStr = thisStepStr
+			} else {
+				log.Printf("[TRACE] Completed graph transform %T (no changes)", step)
+			}
 		}
 
 		if err != nil {
@@ -65,6 +68,11 @@ func (b *BasicGraphBuilder) Build(path addrs.ModuleInstance) (*Graph, tfdiags.Di
 		}
 	}
 
+	if !logging.GraphTrace {
+		// print the graph at least once for normal trace logs
+		log.Printf("[TRACE] Completed graph transform:\n%s  ------", g.StringWithNodeTypes())
+	}
+
 	// Return early if the graph validation is skipped
 	// This behavior is currently only used by the graph command
 	// which only wants to display the dot representation of the graph