Don't allow a JSModule to be used in more than one JSModuleGraph.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154083550

src/com/google/javascript/jscomp/Compiler.java

@@ -126,8 +126,6 @@ public class Compiler extends AbstractCompiler implements ErrorHandler, SourceFi
   // The JS source modules
   private List<JSModule> modules;
 
-  // The graph of the JS source modules. Must be null if there are less than
-  // 2 modules, because we use this as a signal for which passes to run.
   private JSModuleGraph moduleGraph;
 
   // The module loader for resolving paths into module URIs.
@@ -536,18 +534,14 @@ public <T extends SourceFile> void initModules(
     // Generate the module graph, and report any errors in the module
     // specification as errors.
     this.modules = modules;
-    if (modules.size() > 1) {
-      try {
-        this.moduleGraph = new JSModuleGraph(modules);
-      } catch (JSModuleGraph.ModuleDependenceException e) {
-        // problems with the module format.  Report as an error.  The
-        // message gives all details.
-        report(JSError.make(MODULE_DEPENDENCY_ERROR,
-                e.getModule().getName(), e.getDependentModule().getName()));
-        return;
-      }
-    } else {
-      this.moduleGraph = null;
+    try {
+      this.moduleGraph = new JSModuleGraph(modules);
+    } catch (JSModuleGraph.ModuleDependenceException e) {
+      // problems with the module format.  Report as an error.  The
+      // message gives all details.
+      report(JSError.make(MODULE_DEPENDENCY_ERROR,
+          e.getModule().getName(), e.getDependentModule().getName()));
+      return;
     }
 
     this.inputs = getAllInputsFromModules(modules);
@@ -559,6 +553,16 @@ public <T extends SourceFile> void initModules(
     initAST();
   }
 
+  /**
+   * Exists only for some tests that want to reuse JSModules.
+   * @deprecated Fix those tests.
+   */
+  @Deprecated
+  public void breakThisCompilerSoItsModulesCanBeReused() {
+    moduleGraph.breakThisGraphSoItsModulesCanBeReused();
+    moduleGraph = null;
+  }
+
   /**
    * Do any initialization that is dependent on the compiler options.
    */
@@ -1342,17 +1346,28 @@ boolean addNewSourceAst(JsAst ast) {
     return true;
   }
 
+  /**
+   * The graph of the JS source modules.
+   *
+   * <p>Must return null if there are less than 2 modules,
+   * because we use this as a signal for which passes to run.
+   * TODO(bradfordcsmith): Just check for a single module instead of null.
+   */
   @Override
   JSModuleGraph getModuleGraph() {
-    return moduleGraph;
+    if (moduleGraph != null && modules.size() > 1) {
+      return moduleGraph;
+    } else {
+      return null;
+    }
   }
 
   /**
    * Gets a module graph. This will always return a module graph, even
    * in the degenerate case when there's only one module.
    */
   JSModuleGraph getDegenerateModuleGraph() {
-    return moduleGraph == null ? new JSModuleGraph(modules) : moduleGraph;
+    return moduleGraph;
   }
 
   @Override

src/com/google/javascript/jscomp/JSModule.java

@@ -16,6 +16,8 @@
 
 package com.google.javascript.jscomp;
 
+import static com.google.common.base.Preconditions.checkArgument;
+
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
@@ -240,13 +242,18 @@ public String toString() {
   }
 
   /**
-   * Removes any references to nodes of the AST.  This method is needed to
-   * allow the ASTs to be garbage collected if the modules are kept around.
+   * Removes any references to nodes of the AST and resets fields used by JSModuleGraph.
+   *
+   * <p>This method is needed by some tests to allow modules to be reused and their ASTs garbage
+   * collected.
+   * @deprecated Fix tests to avoid reusing modules.
    */
-  public void clearAsts() {
+  @Deprecated
+  void resetThisModuleSoItCanBeReused() {
     for (CompilerInput input : inputs) {
       input.clearAst();
     }
+    depth = -1;
   }
 
   /**
@@ -269,6 +276,7 @@ public void sortInputsByDeps(AbstractCompiler compiler) {
    * @param dep the depth to set
    */
   public void setDepth(int dep) {
+    checkArgument(dep >= 0, "invalid depth: %s", dep);
     this.depth = dep;
   }
 

src/com/google/javascript/jscomp/JSModuleGraph.java

@@ -16,6 +16,8 @@
 
 package com.google.javascript.jscomp;
 
+import static com.google.common.base.Preconditions.checkState;
+
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
@@ -88,6 +90,7 @@ public JSModuleGraph(List<JSModule> modulesInDepOrder) {
     modulesByDepth = new ArrayList<>();
 
     for (JSModule module : modulesInDepOrder) {
+      checkState(module.getDepth() == -1, "Module already used in another graph: %s", module);
       int depth = 0;
       for (JSModule dep : module.getDependencies()) {
         int depDepth = dep.getDepth();
@@ -108,6 +111,17 @@ public JSModuleGraph(List<JSModule> modulesInDepOrder) {
     }
   }
 
+  /**
+   * This only exists as a temprorary workaround.
+   * @deprecated Fix the tests that use this.
+   */
+  @Deprecated
+  public void breakThisGraphSoItsModulesCanBeReused() {
+    for (JSModule m : modules) {
+      m.resetThisModuleSoItCanBeReused();
+    }
+  }
+
   /**
    * Gets an iterable over all modules in dependency order.
    */