Move the android logging doc to //docs

docs/android_logging.md

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+# Logging #

[Bernhard Bauer, 2015/09/02 10:40:15]

I wonder why this doesn't show up as a move...

[dgn, 2015/09/02 10:57:50]

I did a bunch of changes to make it 80 char lines, specify the language for the
snippets, etc. The similarity is probably very low

[dgn, 2015/09/02 11:03:10]

shows as move with `git cl upload --similarity 10`!

+
+[TOC]
+
+
+## Overview
+
+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:
+
+```java
+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.toString
+```
+
+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:
+
+```java
+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
+
+```java
+// 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.
+
+```java
+static private final boolean DEBUG = false;  // debug toggle.
+...
+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.
+
+```java
+/* 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:
+
+```java
+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:
+
+```java
+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:
+
+```java
+Log.d(TAG, "fields [%s,%s,%s]", value1, value2, value3);
+```
+
+## Filtering logs
+
+Logcat allows filtering by specifying tags and the associated level:
+
+```shell
+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:
+
+```shell
+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)