From 9993dfafa278f314088930cff70ffdd8de94562a Mon Sep 17 00:00:00 2001
From: Thulinma <jaron@vietors.com>
Date: Mon, 21 Mar 2022 13:25:55 +0100
Subject: [PATCH] Implemented exponential backoff for shared memory related
 waits

---
 lib/shared_memory.cpp | 16 ++++++++--------
 lib/util.cpp          | 15 +++++++++++++++
 lib/util.h            |  2 ++
 3 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/lib/shared_memory.cpp b/lib/shared_memory.cpp
index 272a132c..0cc78f19 100644
--- a/lib/shared_memory.cpp
+++ b/lib/shared_memory.cpp
@@ -100,7 +100,7 @@ namespace IPC{
       }
       if (!(*this)){
         if (GetLastError() == ERROR_FILE_NOT_FOUND && !noWait){// Error code 2
-          Util::wait(500);
+          Util::wait(Util::expBackoffMs(timer-1, 10, 5000));
         }else{
           break;
         }
@@ -121,7 +121,7 @@ namespace IPC{
       }
       if (!(*this)){
         if (errno == ENOENT && !noWait){
-          Util::wait(500);
+          Util::wait(Util::expBackoffMs(timer-1, 10, 5000));
         }else{
           break;
         }
@@ -458,10 +458,10 @@ namespace IPC{
       }else{
         int i = 0;
         do{
-          if (i != 0){Util::wait(1000);}
+          if (i != 0){Util::wait(Util::expBackoffMs(i-1, 10, 10000));}
           handle = OpenFileMappingA(FILE_MAP_ALL_ACCESS, FALSE, name.c_str());
           i++;
-        }while (i < 10 && !handle && autoBackoff);
+        }while (i <= 10 && !handle && autoBackoff);
       }
       if (!handle){
         MEDIUM_MSG("%s for page %s failed with error code %u",
@@ -489,9 +489,9 @@ namespace IPC{
           handle = shm_open(name.c_str(), O_CREAT | O_RDWR, ACCESSPERMS);
         }else{
           int i = 0;
-          while (i < 10 && handle == -1 && autoBackoff){
+          while (i < 11 && handle == -1 && autoBackoff){
             i++;
-            Util::wait(1000);
+            Util::wait(Util::expBackoffMs(i-1, 10, 10000));
             handle = shm_open(name.c_str(), O_RDWR, ACCESSPERMS);
           }
         }
@@ -620,9 +620,9 @@ namespace IPC{
                         O_CREAT | O_TRUNC | O_RDWR, (mode_t)0600);
         }else{
           int i = 0;
-          while (i < 10 && handle == -1 && autoBackoff){
+          while (i < 11 && handle == -1 && autoBackoff){
             i++;
-            Util::wait(1000);
+            Util::wait(Util::expBackoffMs(i-1, 10, 10000));
             handle = open(std::string(Util::getTmpFolder() + name).c_str(), O_RDWR, (mode_t)0600);
           }
         }
diff --git a/lib/util.cpp b/lib/util.cpp
index bd324272..852a6f4d 100644
--- a/lib/util.cpp
+++ b/lib/util.cpp
@@ -149,6 +149,21 @@ namespace Util{
     }
   }
 
+  //Returns the time to wait in milliseconds for exponential back-off waiting.
+  //If currIter > maxIter, always returns 5ms to prevent tight eternal loops when mistakes are made
+  //Otherwise, exponentially increases wait time for a total of maxWait milliseconds after maxIter calls.
+  //Wildly inaccurate for very short max durations and/or very large maxIter values, but otherwise works as expected.
+  int64_t expBackoffMs(const size_t currIter, const size_t maxIter, const int64_t maxWait){
+    if (currIter > maxIter){return 5;}
+    int64_t w = maxWait >> 1;
+    for (size_t i = maxIter; i > currIter; --i){
+      w >>= 1;
+      if (w < 2){w = 2;}
+    }
+    DONTEVEN_MSG("Waiting %" PRId64 " ms out of %" PRId64 " for iteration %zu/%zu", w, maxWait, currIter, maxIter);
+    return w;
+  }
+
   /// 64-bits version of ftell
   uint64_t ftell(FILE *stream){
     /// \TODO Windows implementation (e.g. _ftelli64 ?)
diff --git a/lib/util.h b/lib/util.h
index af77abf2..c19f32e1 100644
--- a/lib/util.h
+++ b/lib/util.h
@@ -14,6 +14,8 @@ namespace Util{
   bool stringScan(const std::string &src, const std::string &pattern, std::deque<std::string> &result);
   void stringToLower(std::string &val);
 
+  int64_t expBackoffMs(const size_t currIter, const size_t maxIter, const int64_t maxWait);
+
   uint64_t ftell(FILE *stream);
   uint64_t fseek(FILE *stream, uint64_t offset, int whence);