Merge #18817: doc: Document differences in bitcoind and bitcoin-qt locale handling

ca185cf5a1 doc: Document differences in bitcoind and bitcoin-qt locale handling (practicalswift)

Pull request description:

  Document differences in `bitcoind` and `bitcoin-qt` locale handling.

  Since this seems to be the root cause to the locale dependency issues we've seen over the years I thought it was worth documenting :)

  Note that 1.) `QLocale` (used by Qt), 2.) C locale (used by locale-sensitive C standard library functions/POSIX functions and some parts of the C++ standard library such as `std::to_string`) and 3.) C++ locale (used by the C++ input/output library) are three separate things. This comment is about the perhaps surprising interference with the C locale (2) that takes place as part of the Qt initialization.

ACKs for top commit:
  hebasto:
    re-ACK ca185cf5a1

Tree-SHA512: e51c32f3072c506b0029a001d8b108125e1acb4f2b6a48a6be721ddadda9da0ae77a9b39ff33f9d9eebabe2244c1db09e8502e3e7012d7a5d40d98e96da0dc44

src/qt/bitcoin.cpp

@@ -206,6 +206,7 @@ BitcoinApplication::BitcoinApplication():
     returnValue(0),
     platformStyle(nullptr)
 {
+    // Qt runs setlocale(LC_ALL, "") on initialization.
     RegisterMetaTypes();
     setQuitOnLastWindowClosed(false);
 }

test/lint/lint-locale-dependence.sh

@@ -4,6 +4,39 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 export LC_ALL=C
+
+# Be aware that bitcoind and bitcoin-qt differ in terms of localization: Qt
+# opts in to POSIX localization by running setlocale(LC_ALL, "") on startup,
+# whereas no such call is made in bitcoind.
+#
+# Qt runs setlocale(LC_ALL, "") on initialization. This installs the locale
+# specified by the user's LC_ALL (or LC_*) environment variable as the new
+# C locale.
+#
+# In contrast, bitcoind does not opt in to localization -- no call to
+# setlocale(LC_ALL, "") is made and the environment variables LC_* are
+# thus ignored.
+#
+# This results in situations where bitcoind is guaranteed to be running
+# with the classic locale ("C") whereas the locale of bitcoin-qt will vary
+# depending on the user's environment variables.
+#
+# An example: Assuming the environment variable LC_ALL=de_DE then the
+# call std::to_string(1.23) will return "1.230000" in bitcoind but
+# "1,230000" in bitcoin-qt.
+#
+# From the Qt documentation:
+# "On Unix/Linux Qt is configured to use the system locale settings by default.
+#  This can cause a conflict when using POSIX functions, for instance, when
+#  converting between data types such as floats and strings, since the notation
+#  may differ between locales. To get around this problem, call the POSIX function
+#  setlocale(LC_NUMERIC,"C") right after initializing QApplication, QGuiApplication
+#  or QCoreApplication to reset the locale that is used for number formatting to
+#  "C"-locale."
+#
+# See https://doc.qt.io/qt-5/qcoreapplication.html#locale-settings and
+# https://stackoverflow.com/a/34878283 for more details.
+
 KNOWN_VIOLATIONS=(
     "src/bitcoin-tx.cpp.*stoul"
     "src/bitcoin-tx.cpp.*trim_right"