Decrease size of LogBufferElements to 32 bytes.

- Remove virtual from the destructor.
- Remove mTag and derive it when calling getTag().
- Add a boolean mDropped to tell when a message is dropped.
- When dropping a message, and it contains valid tag data, reallocate
  the message to only contain the tag data.
- Add the packed tag to the class.

This saves about ~150K of PSS on a typical log stream since it moves the
size of the LogBufferElement from 48 bytes to 32 bytes which puts it in
a smaller bin.

Bug: 63792187

Test: Builds, unit tests pass.
Change-Id: Ia5afce343ea3d344fcecd78c648338e94f5c9312

logd/LogBufferElement.cpp

@@ -41,22 +41,20 @@ LogBufferElement::LogBufferElement(log_id_t log_id, log_time realtime,
       mTid(tid),
       mRealTime(realtime),
       mMsgLen(len),
-      mLogId(log_id) {
+      mLogId(log_id),
+      mDropped(false) {
     mMsg = new char[len];
     memcpy(mMsg, msg, len);
-    mTag = (isBinary() && (mMsgLen >= sizeof(uint32_t)))
-               ? le32toh(reinterpret_cast<android_event_header_t*>(mMsg)->tag)
-               : 0;
 }
 
 LogBufferElement::LogBufferElement(const LogBufferElement& elem)
-    : mTag(elem.mTag),
-      mUid(elem.mUid),
+    : mUid(elem.mUid),
       mPid(elem.mPid),
       mTid(elem.mTid),
       mRealTime(elem.mRealTime),
       mMsgLen(elem.mMsgLen),
-      mLogId(elem.mLogId) {
+      mLogId(elem.mLogId),
+      mDropped(elem.mDropped) {
     mMsg = new char[mMsgLen];
     memcpy(mMsg, elem.mMsg, mMsgLen);
 }
@@ -65,6 +63,32 @@ LogBufferElement::~LogBufferElement() {
     delete[] mMsg;
 }
 
+uint32_t LogBufferElement::getTag() const {
+    return (isBinary() &&
+            ((mDropped && mMsg != nullptr) ||
+             (!mDropped && mMsgLen >= sizeof(android_event_header_t))))
+               ? reinterpret_cast<const android_event_header_t*>(mMsg)->tag
+               : 0;
+}
+
+unsigned short LogBufferElement::setDropped(unsigned short value) {
+    // The tag information is saved in mMsg data, if the tag is non-zero
+    // save only the information needed to get the tag.
+    if (getTag() != 0) {
+        if (mMsgLen > sizeof(android_event_header_t)) {
+            char* truncated_msg = new char[sizeof(android_event_header_t)];
+            memcpy(truncated_msg, mMsg, sizeof(android_event_header_t));
+            delete[] mMsg;
+            mMsg = truncated_msg;
+        }  // mMsgLen == sizeof(android_event_header_t), already at minimum.
+    } else {
+        delete[] mMsg;
+        mMsg = nullptr;
+    }
+    mDropped = true;
+    return mDroppedCount = value;
+}
+
 // caller must own and free character string
 char* android::tidToName(pid_t tid) {
     char* retval = NULL;
@@ -164,8 +188,8 @@ size_t LogBufferElement::populateDroppedMessage(char*& buffer, LogBuffer* parent
     // identical to below to calculate the buffer size required
     const char* type = lastSame ? "identical" : "expire";
     size_t len = snprintf(NULL, 0, format_uid, mUid, name ? name : "",
-                          commName ? commName : "", type, mDropped,
-                          (mDropped > 1) ? "s" : "");
+                          commName ? commName : "", type, getDropped(),
+                          (getDropped() > 1) ? "s" : "");
 
     size_t hdrLen;
     if (isBinary()) {
@@ -196,8 +220,8 @@ size_t LogBufferElement::populateDroppedMessage(char*& buffer, LogBuffer* parent
     }
 
     snprintf(buffer + hdrLen, len + 1, format_uid, mUid, name ? name : "",
-             commName ? commName : "", type, mDropped,
-             (mDropped > 1) ? "s" : "");
+             commName ? commName : "", type, getDropped(),
+             (getDropped() > 1) ? "s" : "");
     free(const_cast<char*>(name));
     free(const_cast<char*>(commName));
 
@@ -225,7 +249,7 @@ log_time LogBufferElement::flushTo(SocketClient* reader, LogBuffer* parent,
 
     char* buffer = NULL;
 
-    if (!mMsg) {
+    if (mDropped) {
         entry.len = populateDroppedMessage(buffer, parent, lastSame);
         if (!entry.len) return mRealTime;
         iovec[1].iov_base = buffer;

logd/LogBufferElement.h

@@ -32,25 +32,25 @@ class LogBuffer;
                                   // chatty for the temporal expire messages
 #define EXPIRE_RATELIMIT 10  // maximum rate in seconds to report expiration
 
-class LogBufferElement {
+class __attribute__((packed)) LogBufferElement {
     friend LogBuffer;
 
     // sized to match reality of incoming log packets
-    uint32_t mTag;  // only valid for isBinary()
     const uint32_t mUid;
     const uint32_t mPid;
     const uint32_t mTid;
     log_time mRealTime;
     char* mMsg;
     union {
-        const uint16_t mMsgLen;  // mMSg != NULL
-        uint16_t mDropped;       // mMsg == NULL
+        const uint16_t mMsgLen;  // mDropped == false
+        uint16_t mDroppedCount;  // mDropped == true
     };
     const uint8_t mLogId;
+    bool mDropped;
 
     static atomic_int_fast64_t sequence;
 
-    // assumption: mMsg == NULL
+    // assumption: mDropped == true
     size_t populateDroppedMessage(char*& buffer, LogBuffer* parent,
                                   bool lastSame);
 
@@ -58,7 +58,7 @@ class LogBufferElement {
     LogBufferElement(log_id_t log_id, log_time realtime, uid_t uid, pid_t pid,
                      pid_t tid, const char* msg, unsigned short len);
     LogBufferElement(const LogBufferElement& elem);
-    virtual ~LogBufferElement();
+    ~LogBufferElement();
 
     bool isBinary(void) const {
         return (mLogId == LOG_ID_EVENTS) || (mLogId == LOG_ID_SECURITY);
@@ -76,24 +76,16 @@ class LogBufferElement {
     pid_t getTid(void) const {
         return mTid;
     }
-    uint32_t getTag() const {
-        return mTag;
-    }
+    uint32_t getTag() const;
     unsigned short getDropped(void) const {
-        return mMsg ? 0 : mDropped;
-    }
-    unsigned short setDropped(unsigned short value) {
-        if (mMsg) {
-            delete[] mMsg;
-            mMsg = NULL;
-        }
-        return mDropped = value;
+        return mDropped ? mDroppedCount : 0;
     }
+    unsigned short setDropped(unsigned short value);
     unsigned short getMsgLen() const {
-        return mMsg ? mMsgLen : 0;
+        return mDropped ? 0 : mMsgLen;
     }
     const char* getMsg() const {
-        return mMsg;
+        return mDropped ? nullptr : mMsg;
     }
     log_time getRealTime(void) const {
         return mRealTime;