fs_mgr: consolidate DT compatible check

Fixes the compatible check in fs_mgr_boot_config by consolidating the
check in a single privately exported function within fs_mgr (i.e.
is_dt_compatible()).

b/27805372

Test: Boot sailfish w/ early mount

Change-Id: Ie2d1646b81cf9eba8d16828ca8cf2c75156c294c
Signed-off-by: Sandeep Patil <sspatil@google.com>

fs_mgr/fs_mgr_boot_config.cpp

@@ -49,15 +49,8 @@ bool fs_mgr_get_boot_config(const std::string& key, std::string* out_val) {
     }
 
     // lastly, check the device tree
-    std::string file_name = kAndroidDtDir + "/compatible";
-    std::string dt_value;
-    if (android::base::ReadFileToString(file_name, &dt_value)) {
-        if (dt_value != "android,firmware") {
-            LERROR << "Error finding compatible android DT node";
-            return false;
-        }
-
-        file_name = kAndroidDtDir + "/" + key;
+    if (is_dt_compatible()) {
+        std::string file_name = kAndroidDtDir + "/" + key;
         // DT entries terminate with '\0' but so do the properties
         if (android::base::ReadFileToString(file_name, out_val)) {
             return true;

fs_mgr/fs_mgr_fstab.cpp

@@ -295,21 +295,6 @@ static int parse_flags(char *flags, struct flag_list *fl,
     return f;
 }
 
-static bool is_dt_compatible() {
-    std::string file_name = kAndroidDtDir + "/compatible";
-    std::string dt_value;
-    if (android::base::ReadFileToString(file_name, &dt_value)) {
-        // trim the trailing '\0' out, otherwise the comparison
-        // will produce false-negatives.
-        dt_value.resize(dt_value.size() - 1);
-        if (dt_value == "android,firmware") {
-            return true;
-        }
-    }
-
-    return false;
-}
-
 static bool is_dt_fstab_compatible() {
     std::string dt_value;
     std::string file_name = kAndroidDtDir + "/fstab/compatible";
@@ -398,6 +383,22 @@ static std::string read_fstab_from_dt() {
     return fstab;
 }
 
+bool is_dt_compatible() {
+    std::string file_name = kAndroidDtDir + "/compatible";
+    std::string dt_value;
+    if (android::base::ReadFileToString(file_name, &dt_value)) {
+        if (!dt_value.empty()) {
+            // trim the trailing '\0' out, otherwise the comparison
+            // will produce false-negatives.
+            dt_value.resize(dt_value.size() - 1);
+            if (dt_value == "android,firmware") {
+                return true;
+            }
+        }
+    }
+
+    return false;
+}
 
 struct fstab *fs_mgr_read_fstab_file(FILE *fstab_file)
 {

fs_mgr/fs_mgr_priv.h

@@ -117,6 +117,7 @@ __BEGIN_DECLS
 int fs_mgr_set_blk_ro(const char *blockdev);
 int fs_mgr_test_access(const char *device);
 int fs_mgr_update_for_slotselect(struct fstab *fstab);
+bool is_dt_compatible();
 
 __END_DECLS