Allow users to set log flushing rules (#20817)

* Add flushThreshold to std/logging loggers

* Remove duplicate field

* Add -d:nimFlushAllLogs for changing default flush behavior globally

* Add changelog entry for log flushing change

* Flush all log levels by default in Nim v2

2 files changed, 38 insertions, 17 deletions

diff --git a/changelog.md b/changelog.md
index b40a5b63e..16bafd8a2 100644
--- a/changelog.md
+++ b/changelog.md
@@ -100,6 +100,8 @@
   This warning will become an error in future versions! Use a `cast` operation
   like `cast[cstring](x)` instead.
 
+- `logging` will default to flushing all log level messages. To get the legacy behaviour of only flushing Error and Fatal messages, use `-d:nimV1LogFlushBehavior`.
+
 ## Standard library additions and changes
 
 [//]: # "Changes:"
@@ -119,6 +121,7 @@
 - `std/oids` now uses `int64` to store time internally (before it was int32).
 - `std/uri.Uri` dollar `$` improved, precalculates the `string` result length from the `Uri`.
 - `std/uri.Uri.isIpv6` is now exported.
+- `std/logging.ConsoleLogger` and `FileLogger` now have a `flushThreshold` attribute to set what log message levels are automatically flushed. For Nim v1 use `-d:nimFlushAllLogs` to automatically flush all message levels. Flushing all logs is the default behavior for Nim v2.
 
 
 - `std/net.IpAddress` dollar `$` improved, uses a fixed capacity for the `string` result based from the `IpAddressFamily`.
diff --git a/lib/pure/logging.nim b/lib/pure/logging.nim
index cbe8a827a..46b59b750 100644
--- a/lib/pure/logging.nim
+++ b/lib/pure/logging.nim
@@ -47,9 +47,8 @@
 ##
 ## .. warning::
 ##   For loggers that log to a console or to files, only error and fatal
-##   messages will cause their output buffers to be flushed immediately.
-##   Use the `flushFile proc <syncio.html#flushFile,File>`_ to flush the buffer
-##   manually if needed.
+##   messages will cause their output buffers to be flushed immediately by default.
+##   set ``flushThreshold`` when creating the logger to change this.
 ##
 ## Handlers
 ## --------
@@ -200,6 +199,15 @@ const
   ## If a different format string is preferred, refer to the
   ## `documentation about format strings<#basic-usage-format-strings>`_
   ## for more information, including a list of available variables.
+  defaultFlushThreshold = when NimMajor >= 2:
+      when defined(nimV1LogFlushBehavior): lvlError else: lvlAll
+    else:
+      when defined(nimFlushAllLogs): lvlAll else: lvlError
+  ## The threshold above which log messages to file-like loggers
+  ## are automatically flushed.
+  ## 
+  ## By default, only error and fatal messages are logged,
+  ## but defining ``-d:nimFlushAllLogs`` will make all levels be flushed
 
 type
   Logger* = ref object of RootObj
@@ -228,6 +236,8 @@ type
     ## * `FileLogger<#FileLogger>`_
     ## * `RollingFileLogger<#RollingFileLogger>`_
     useStderr*: bool ## If true, writes to stderr; otherwise, writes to stdout
+    flushThreshold*: Level ## Only messages that are at or above this
+                           ## threshold will be flushed immediately
 
 when not defined(js):
   type
@@ -243,6 +253,8 @@ when not defined(js):
       ## * `ConsoleLogger<#ConsoleLogger>`_
       ## * `RollingFileLogger<#RollingFileLogger>`_
       file*: File ## The wrapped file
+      flushThreshold*: Level ## Only messages that are at or above this
+                           ## threshold will be flushed immediately
 
     RollingFileLogger* = ref object of FileLogger
       ## A logger that writes log messages to a file while performing log
@@ -348,8 +360,8 @@ method log*(logger: ConsoleLogger, level: Level, args: varargs[string, `$`]) =
   ## `setLogFilter proc<#setLogFilter,Level>`_.
   ##
   ## **Note:** Only error and fatal messages will cause the output buffer
-  ## to be flushed immediately. Use the `flushFile proc
-  ## <syncio.html#flushFile,File>`_ to flush the buffer manually if needed.
+  ## to be flushed immediately by default. Set ``flushThreshold`` when creating
+  ## the logger to change this.
   ##
   ## See also:
   ## * `log method<#log.e,FileLogger,Level,varargs[string,]>`_
@@ -380,12 +392,12 @@ method log*(logger: ConsoleLogger, level: Level, args: varargs[string, `$`]) =
         if logger.useStderr:
           handle = stderr
         writeLine(handle, ln)
-        if level in {lvlError, lvlFatal}: flushFile(handle)
+        if level >= logger.flushThreshold: flushFile(handle)
       except IOError:
         discard
 
 proc newConsoleLogger*(levelThreshold = lvlAll, fmtStr = defaultFmtStr,
-    useStderr = false): ConsoleLogger =
+    useStderr = false, flushThreshold = defaultFlushThreshold): ConsoleLogger =
   ## Creates a new `ConsoleLogger<#ConsoleLogger>`_.
   ##
   ## By default, log messages are written to ``stdout``. If ``useStderr`` is
@@ -409,6 +421,7 @@ proc newConsoleLogger*(levelThreshold = lvlAll, fmtStr = defaultFmtStr,
   new result
   result.fmtStr = fmtStr
   result.levelThreshold = levelThreshold
+  result.flushThreshold = flushThreshold
   result.useStderr = useStderr
 
 when not defined(js):
@@ -424,8 +437,8 @@ when not defined(js):
     ##
     ## **Notes:**
     ## * Only error and fatal messages will cause the output buffer
-    ##   to be flushed immediately. Use the `flushFile proc
-    ##   <syncio.html#flushFile,File>`_ to flush the buffer manually if needed.
+    ##   to be flushed immediately by default. Set ``flushThreshold`` when creating
+    ##   the logger to change this.
     ## * This method is not available for the JavaScript backend.
     ##
     ## See also:
@@ -443,7 +456,7 @@ when not defined(js):
     ##   fileLog.log(lvlError, "error code is: ", 404)
     if level >= logging.level and level >= logger.levelThreshold:
       writeLine(logger.file, substituteLog(logger.fmtStr, level, args))
-      if level in {lvlError, lvlFatal}: flushFile(logger.file)
+      if level >= logger.flushThreshold: flushFile(logger.file)
 
   proc defaultFilename*(): string =
     ## Returns the filename that is used by default when naming log files.
@@ -454,7 +467,8 @@ when not defined(js):
 
   proc newFileLogger*(file: File,
                       levelThreshold = lvlAll,
-                      fmtStr = defaultFmtStr): FileLogger =
+                      fmtStr = defaultFmtStr,
+                      flushThreshold = defaultFlushThreshold): FileLogger =
     ## Creates a new `FileLogger<#FileLogger>`_ that uses the given file handle.
     ##
     ## **Note:** This proc is not available for the JavaScript backend.
@@ -478,13 +492,15 @@ when not defined(js):
     new(result)
     result.file = file
     result.levelThreshold = levelThreshold
+    result.flushThreshold = flushThreshold
     result.fmtStr = fmtStr
 
   proc newFileLogger*(filename = defaultFilename(),
                       mode: FileMode = fmAppend,
                       levelThreshold = lvlAll,
                       fmtStr = defaultFmtStr,
-                      bufSize: int = -1): FileLogger =
+                      bufSize: int = -1,
+                      flushThreshold = defaultFlushThreshold): FileLogger =
     ## Creates a new `FileLogger<#FileLogger>`_ that logs to a file with the
     ## given filename.
     ##
@@ -508,7 +524,7 @@ when not defined(js):
     ##   var formatLog = newFileLogger("formatted.log", fmtStr=verboseFmtStr)
     ##   var errorLog = newFileLogger("errors.log", levelThreshold=lvlError)
     let file = open(filename, mode, bufSize = bufSize)
-    newFileLogger(file, levelThreshold, fmtStr)
+    newFileLogger(file, levelThreshold, fmtStr, flushThreshold)
 
   # ------
 
@@ -540,7 +556,8 @@ when not defined(js):
                             levelThreshold = lvlAll,
                             fmtStr = defaultFmtStr,
                             maxLines: Positive = 1000,
-                            bufSize: int = -1): RollingFileLogger =
+                            bufSize: int = -1,
+                            flushThreshold = defaultFlushThreshold): RollingFileLogger =
     ## Creates a new `RollingFileLogger<#RollingFileLogger>`_.
     ##
     ## Once the current log file being written to contains ``maxLines`` lines,
@@ -576,6 +593,7 @@ when not defined(js):
     result.curLine = 0
     result.baseName = filename
     result.baseMode = mode
+    result.flushThreshold = flushThreshold
 
     result.logFiles = countFiles(filename)
 
@@ -602,8 +620,8 @@ when not defined(js):
     ##
     ## **Notes:**
     ## * Only error and fatal messages will cause the output buffer
-    ##   to be flushed immediately. Use the `flushFile proc
-    ##   <syncio.html#flushFile,File>`_ to flush the buffer manually if needed.
+    ##   to be flushed immediately by default. Set ``flushThreshold`` when creating
+    ##   the logger to change this.
     ## * This method is not available for the JavaScript backend.
     ##
     ## See also:
@@ -629,7 +647,7 @@ when not defined(js):
             bufSize = logger.bufSize)
 
       writeLine(logger.file, substituteLog(logger.fmtStr, level, args))
-      if level in {lvlError, lvlFatal}: flushFile(logger.file)
+      if level >= logger.flushThreshold: flushFile(logger.file)
       logger.curLine.inc
 
 # --------