Move the android logging doc to //docs

base/android/java/src/org/chromium/base/README_logging.md

Index: base/android/java/src/org/chromium/base/README_logging.md
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-## Logging ##
-
-Logging used to be done using Android's [android.util.Log]
-(http://developer.android.com/reference/android/util/Log.html).
-
-A wrapper on that is now available: org.chromium.base.Log. It is designed to write logs as
-belonging to logical groups going beyond single classes, and to make it easy to switch logging on
-or off for individual groups.
-
-Usage:
-
-    private static final String TAG = "cr.YourModuleTag";
-    ...
-    Log.i(TAG, "Logged INFO message.");
-    Log.d(TAG, "Some DEBUG info: %s", data);
-
-Output:
-
-    I/cr.YourModuleTag: ( 999): Logged INFO message
-    D/cr.YourModuleTag: ( 999): [MyClass.java:42] Some DEBUG info: data's toString output
-
-Here, **TAG** will be a feature or package name, "MediaRemote" or "NFC" for example. In most
-cases, the class name is not needed.
-
-### Verbose and Debug logs have special handling ###
-
-*   `Log.v` and `Log.d` Calls made using `org.chromium.base.Log` are stripped
-    out of production binaries using Proguard. There is no way to get those logs
-    in release builds.
-
-*   The file name and line number will be prepended to the log message.
-    For higher priority logs, those are not added for performance concerns.
-
-### An exception trace is printed when the exception is the last parameter ###
-
-As with `java.util.Log`, putting a throwable as last parameter will dump the corresponding stack
-trace:
-
-    Log.i(TAG, "An error happened: %s", e)
-
-    I/cr.YourModuleTag: ( 999): An error happened: This is the exception's message
-    I/cr.YourModuleTag: ( 999): java.lang.Exception: This is the exception's message
-    I/cr.YourModuleTag: ( 999):     at foo.bar.MyClass.test(MyClass.java:42)
-    I/cr.YourModuleTag: ( 999):     ...
-
-Having the exception as last parameter doesn't prevent it from being used for string formatting.
-
-### Logging Best Practices
-
-#### Rule #1: Never log PII (Personal Identification Information):
-
-This is a huge concern, because other applications can access the log and extract a lot of data
-from your own by doing so. Even if JellyBean restricted this, people are going to run your
-application on rooted devices and allow some apps to access it. Also anyone with USB access to the
-device can use ADB to get the full logcat and get the same data right now.
-
-If you really need to print something , print a series of Xs instead (e.g. "XXXXXX"), or print a
-truncated hash of the PII instead. Truncation is required to make it harder for an attacker to
-recover the full data through rainbow tables and similar methods.
-
-Similarly, avoid dumping API keys, cookies, etc...
-
-#### Rule #2: Do not build debug logs in production code:
-
-The log methods are removed in release builds using Proguard. Because log messages might not be
-written, the cost of creating them should also be avoided. This can be done using three
-complementary ways:
-
--   Use string formatting instead of concatenations
-
-        // BAD
-        Log.d(TAG, "I " + preference + " writing logs.");
-
-        // BETTER
-        Log.d(TAG, "I %s writing logs.", preference);
-
-    Proguard removes the method call itself, but doesn't do anything about the arguments. The
-    method's arguments will still be computed and provided as input. The first call above will
-    always lead to the creation of a `StringBuilder` and a few concatenations, while the second just
-    passes the arguments and won't need that.
-
--   Guard expensive calls
-
-    Sometimes the values to log aren't readily available and need to be computed specially. This
-    should be avoided when logging is disabled.
-
-        static private final boolean DEBUG = false;  // set to 'true' to enable debug
-        ...
-        if (DEBUG) {
-          Log.i(TAG, createThatExpensiveLogMessage(activity))
-        }
-
-    Because the variable is a `static final` that can be evaluated at compile time, the Java
-    compiler will optimize out all guarded calls from the generated `.class` file. Changing it
-    however requires editing each of the files for which debug should be enabled and recompiling.
-
--   Annotate debug functions with the `@RemovableInRelease` annotation.
-
-    That annotation tells Proguard to assume that a given function has no side effects, and is
-    called only for its returned value. If this value is unused, the call will be removed. If the
-    function is not called at all, it will also be removed. Since Proguard is already used to
-    strip debug and verbose calls out of release builds, this annotation allows it to have a
-    deeper action by removing also function calls used to generate the log call's arguments.
-
-        /* If that function is only used in Log.d calls, proguard should completely remove it from
-         * the release builds. */
-        @RemovableInRelease
-        private static String getSomeDebugLogString(Thing[] things) {
-          StringBuilder sb = new StringBuilder("Reporting " + thing.length + " things:");
-          for (Thing thing : things) {
-            sb.append('\n').append(thing.id).append(' ').append(report.foo);
-          }
-          return sb.toString();
-        }
-
-        public void bar() {
-          ...
-          Log.d(TAG, getSomeDebugLogString(things)); /* The line is removed in release builds. */
-        }
-
-    Again, this is useful only if the input to that function are variables already available in
-    the scope. The idea is to move computations, concatenations, etc. to a place where that can be
-    removed when not needed, without invading the main function's logic. It can then have a similar
-    effect as guarding with a static final property that would be enabled in Debug and disabled in
-    Release.
-
-#### Rule #3: Favor small log messages
-
-This is still related to the global fixed-sized kernel buffer used to keep all logs. Try to make
-your log information as terse as possible. This reduces the risk of pushing interesting log data
-out of the buffer when something really nasty happens. It's really better to have a single-line
-log message, than several ones. I.e. don't use:
-
-    Log.GROUP.d(TAG, "field1 = %s", value1);
-    Log.GROUP.d(TAG, "field2 = %s", value2);
-    Log.GROUP.d(TAG, "field3 = %s", value3);
-
-Instead, write this as:
-
-    Log.d(TAG, "field1 = %s, field2 = %s, field3 = %s", value1, value2, value3);
-
-That doesn't seem to be much different if you count overall character counts, but each independent
-log entry also implies a small, but non-trivial header, in the kernel log buffer.
-And since every byte count, you can also try something even shorter, as in:
-
-    Log.d(TAG, "fields [%s,%s,%s]", value1, value2, value3);
-
-### Filtering logs
-
-Logcat allows filtering by specifying tags and the associated level:
-
-    adb logcat [TAG_EXPR:LEVEL]...
-    adb logcat cr.YourModuleTag:D *:S
-
-This shows only logs having a level higher or equal to DEBUG for `cr.YourModuleTag`, and SILENT
-(nothing is logged at this level or higher, so it silences the tags) for everything else. You can
-persist a filter by setting an environment variable:
-
-    export ANDROID_LOG_TAGS="cr.YourModuleTag:D *:S"
-
-For more, see the [related page on developer.android.com]
-(http://developer.android.com/tools/debugging/debugging-log.html#filteringOutput)