Detect the absence of the default fstab

This is a follow-up of I828ce999be6d786bf46dd5655dfda81d046906ab. The
change introduced a behavioral change that fstab is read twice: before
root is changed to /first_stage_ramdisk, and once again after that.
Previously, that happend only after the root is switched. That change
caused a problem when there is no fstab in DT and fstab is provided via
a file. The fstab file has been at
/first_stage_ramdisk/fstab.<hardware> because that file was supposed to
be read after the root switch.

With the change, init fails to read the fstab during the first attempt
because there is no /fstab.<hardware> at the moment. Here comes the
problem. Although it failed to read fstab, DoCreateService() is invoked
because ReadFirstStageFstab() doesn't report the failure; it returns an
empty fstab object. As a result, DoCreateDevices() is called but it
doesn't create the dm linear device because it couldn't find an fstab
entry having `logical` option.

Then after /first_stage_ramdisk becomes the root, the fstab file is
correctly read. But since the prior run of DoCreateDevices() is recorded
as 'done', init doesn't try to do that again; dm linear device is never
created. Then we fail to mount any of the logical partitions.

This change fixes the problem by modifying ReadFirstStageFstab()
function so that the failure is correctly reported back to the caller.
When it fails, DoCreateDevices() is not called.

Bug: N/A
Test: Watch TH
Change-Id: Idf2dbc6c0fb6c311ab3f5ff1f28315f7daa2b4ce

init/first_stage_mount.cpp

@@ -44,6 +44,7 @@
 
 #include "block_dev_initializer.h"
 #include "devices.h"
+#include "result.h"
 #include "snapuserd_transition.h"
 #include "switch_root.h"
 #include "uevent.h"
@@ -51,6 +52,7 @@
 #include "util.h"
 
 using android::base::ReadFileToString;
+using android::base::Result;
 using android::base::Split;
 using android::base::StringPrintf;
 using android::base::Timer;
@@ -81,7 +83,7 @@ class FirstStageMount {
 
     // The factory method to create either FirstStageMountVBootV1 or FirstStageMountVBootV2
     // based on device tree configurations.
-    static std::unique_ptr<FirstStageMount> Create();
+    static Result<std::unique_ptr<FirstStageMount>> Create();
     bool DoCreateDevices();    // Creates devices and logical partitions from storage devices
     bool DoFirstStageMount();  // Mounts fstab entries read from device tree.
     bool InitDevices();
@@ -160,7 +162,7 @@ static inline bool IsDtVbmetaCompatible(const Fstab& fstab) {
     return is_android_dt_value_expected("vbmeta/compatible", "android,vbmeta");
 }
 
-static Fstab ReadFirstStageFstab() {
+static Result<Fstab> ReadFirstStageFstab() {
     Fstab fstab;
     if (!ReadFstabFromDt(&fstab)) {
         if (ReadDefaultFstab(&fstab)) {
@@ -170,7 +172,7 @@ static Fstab ReadFirstStageFstab() {
                                        }),
                         fstab.end());
         } else {
-            LOG(INFO) << "Failed to fstab for first stage mount";
+            return Error() << "failed to read default fstab for first stage mount";
         }
     }
     return fstab;
@@ -236,12 +238,16 @@ FirstStageMount::FirstStageMount(Fstab fstab) : need_dm_verity_(false), fstab_(s
     super_partition_name_ = fs_mgr_get_super_partition_name();
 }
 
-std::unique_ptr<FirstStageMount> FirstStageMount::Create() {
+Result<std::unique_ptr<FirstStageMount>> FirstStageMount::Create() {
     auto fstab = ReadFirstStageFstab();
-    if (IsDtVbmetaCompatible(fstab)) {
+    if (!fstab.ok()) {
-        return std::make_unique<FirstStageMountVBootV2>(std::move(fstab));
+        return fstab.error();
+    }
+
+    if (IsDtVbmetaCompatible(*fstab)) {
+        return std::make_unique<FirstStageMountVBootV2>(std::move(*fstab));
     } else {
-        return std::make_unique<FirstStageMountVBootV1>(std::move(fstab));
+        return std::make_unique<FirstStageMountVBootV1>(std::move(*fstab));
     }
 }
 
@@ -836,12 +842,12 @@ bool FirstStageMountVBootV2::InitAvbHandle() {
 // ----------------
 // Creates devices and logical partitions from storage devices
 bool DoCreateDevices() {
-    std::unique_ptr<FirstStageMount> handle = FirstStageMount::Create();
+    auto fsm = FirstStageMount::Create();
-    if (!handle) {
+    if (!fsm.ok()) {
-        LOG(ERROR) << "Failed to create FirstStageMount";
+        LOG(ERROR) << "Failed to create FirstStageMount: " << fsm.error();
         return false;
     }
-    return handle->DoCreateDevices();
+    return (*fsm)->DoCreateDevices();
 }
 
 // Mounts partitions specified by fstab in device tree.
@@ -852,17 +858,17 @@ bool DoFirstStageMount(bool create_devices) {
         return true;
     }
 
-    std::unique_ptr<FirstStageMount> handle = FirstStageMount::Create();
+    auto fsm = FirstStageMount::Create();
-    if (!handle) {
+    if (!fsm.ok()) {
-        LOG(ERROR) << "Failed to create FirstStageMount";
+        LOG(ERROR) << "Failed to create FirstStageMount " << fsm.error();
         return false;
     }
 
     if (create_devices) {
-        if (!handle->DoCreateDevices()) return false;
+        if (!(*fsm)->DoCreateDevices()) return false;
     }
 
-    return handle->DoFirstStageMount();
+    return (*fsm)->DoFirstStageMount();
 }
 
 void SetInitAvbVersionInRecovery() {
@@ -872,8 +878,12 @@ void SetInitAvbVersionInRecovery() {
     }
 
     auto fstab = ReadFirstStageFstab();
+    if (!fstab.ok()) {
+        LOG(ERROR) << fstab.error();
+        return;
+    }
 
-    if (!IsDtVbmetaCompatible(fstab)) {
+    if (!IsDtVbmetaCompatible(*fstab)) {
         LOG(INFO) << "Skipped setting INIT_AVB_VERSION (not vbmeta compatible)";
         return;
     }
@@ -883,7 +893,7 @@ void SetInitAvbVersionInRecovery() {
     // We only set INIT_AVB_VERSION when the AVB verification succeeds, i.e., the
     // Open() function returns a valid handle.
     // We don't need to mount partitions here in recovery mode.
-    FirstStageMountVBootV2 avb_first_mount(std::move(fstab));
+    FirstStageMountVBootV2 avb_first_mount(std::move(*fstab));
     if (!avb_first_mount.InitDevices()) {
         LOG(ERROR) << "Failed to init devices for INIT_AVB_VERSION";
         return;