Revert "Force LC_COLLATE to C in postmaster."

Out of an abundance of caution.

This reverts commit 5e6e42e44f.

Requested-by: Daniel Verite <daniel@manitou-mail.org>
Requested-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20260826232534.13.noahmisch@microsoft.com
Backpatch-through: 19

doc/src/sgml/catalogs.sgml

@@ -3187,7 +3187,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        <structfield>datcollate</structfield> <type>text</type>
       </para>
       <para>
-       LC_COLLATE for this database (ignored unless <structfield>datlocprovider</structfield> is <literal>c</literal>)
+       LC_COLLATE for this database
       </para></entry>
      </row>
 

doc/src/sgml/charset.sgml

@@ -100,7 +100,7 @@ initdb --locale=sv_SE
       <tbody>
        <row>
         <entry><envar>LC_COLLATE</envar></entry>
-        <entry>String sort order (ignored unless the provider is <literal>libc</literal>)</entry>
+        <entry>String sort order</entry>
        </row>
        <row>
         <entry><envar>LC_CTYPE</envar></entry>

doc/src/sgml/ref/create_database.sgml

@@ -150,12 +150,12 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
        <para>
         Sets the default collation order and character classification in the
         new database.  Collation affects the sort order applied to strings,
-        e.g., in queries with <literal>ORDER BY</literal>, as well as the
-        order used in indexes on text columns.  Character classification
-        affects the categorization of characters, e.g., lower, upper, and
-        digit.  Also sets the <literal>LC_CTYPE</literal> aspect of the
-        operating system environment.  The default is the same setting as the
-        template database.  See <xref
+        e.g., in queries with <literal>ORDER BY</literal>, as well as the order used in indexes
+        on text columns.  Character classification affects the categorization
+        of characters, e.g., lower, upper, and digit.  Also sets the
+        associated aspects of the operating system environment,
+        <literal>LC_COLLATE</literal> and <literal>LC_CTYPE</literal>.  The
+        default is the same setting as the template database.  See <xref
         linkend="collation-managing-create-libc"/> and <xref
         linkend="collation-managing-create-icu"/> for details.
        </para>
@@ -189,16 +189,17 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
       <term><replaceable class="parameter">lc_collate</replaceable></term>
       <listitem>
        <para>
-        If <xref linkend="create-database-locale-provider"/> is
-        <literal>libc</literal>, sets the default collation order to use in
-        the new database, overriding the setting <xref
-        linkend="create-database-locale"/>. Otherwise, this setting is
-        ignored.
+        Sets <literal>LC_COLLATE</literal> in the database server's operating
+        system environment.  The default is the setting of <xref
+        linkend="create-database-locale"/> if specified, otherwise the same
+        setting as the template database.  See below for additional
+        restrictions.
        </para>
        <para>
-        The default is the setting of <xref linkend="create-database-locale"/>
-        if specified, otherwise the same setting as the template database.
-        See below for additional restrictions.
+        If <xref linkend="create-database-locale-provider"/> is
+        <literal>libc</literal>, also sets the default collation order to use
+        in the new database, overriding the setting <xref
+        linkend="create-database-locale"/>.
        </para>
       </listitem>
      </varlistentry>
@@ -207,18 +208,16 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
       <listitem>
        <para>
         Sets <literal>LC_CTYPE</literal> in the database server's operating
-        system environment.
+        system environment.  The default is the setting of <xref
+        linkend="create-database-locale"/> if specified, otherwise the same
+        setting as the template database.  See below for additional
+        restrictions.
        </para>
        <para>
         If <xref linkend="create-database-locale-provider"/> is
-        <literal>libc</literal>, sets the default character classification to
-        use in the new database, overriding the setting <xref
-        linkend="create-database-locale"/>.
-       </para>
-       <para>
-        The default is the setting of <xref linkend="create-database-locale"/>
-        if specified, otherwise the same setting as the template database.
-        See below for additional restrictions.
+        <literal>libc</literal>, also sets the default character
+        classification to use in the new database, overriding the setting
+        <xref linkend="create-database-locale"/>.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/ref/createdb.sgml

@@ -136,8 +136,7 @@ PostgreSQL documentation
       <term><option>--lc-collate=<replaceable class="parameter">locale</replaceable></option></term>
       <listitem>
        <para>
-        Specifies the LC_COLLATE setting to be used in this database (ignored
-        unless the locale provider is <literal>libc</literal>).
+        Specifies the LC_COLLATE setting to be used in this database.
        </para>
       </listitem>
      </varlistentry>

src/backend/main/main.c

@@ -125,17 +125,13 @@ main(int argc, char *argv[])
 	set_pglocale_pgservice(argv[0], PG_TEXTDOMAIN("postgres"));
 
 	/*
-	 * Collation is handled by pg_locale.c, and the behavior is dependent on
-	 * the provider. strcoll(), etc., should not be called directly.
-	 */
-	init_locale("LC_COLLATE", LC_COLLATE, "C");
-
-	/*
-	 * In the postmaster, absorb the environment value for LC_CTYPE.
-	 * Individual backends will change it later to pg_database.datctype, but
-	 * the postmaster cannot do that.  If we leave it set to "C" then message
-	 * localization might not work well in the postmaster.
+	 * In the postmaster, absorb the environment values for LC_COLLATE and
+	 * LC_CTYPE.  Individual backends will change these later to settings
+	 * taken from pg_database, but the postmaster cannot do that.  If we leave
+	 * these set to "C" then message localization might not work well in the
+	 * postmaster.
 	 */
+	init_locale("LC_COLLATE", LC_COLLATE, "");
 	init_locale("LC_CTYPE", LC_CTYPE, "");
 
 	/*

src/backend/utils/init/postinit.c

@@ -432,11 +432,12 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
 	datum = SysCacheGetAttrNotNull(DATABASEOID, tup, Anum_pg_database_datctype);
 	ctype = TextDatumGetCString(datum);
 
-	/*
-	 * Historically, we set LC_COLLATE from datcollate, as well. That's no
-	 * longer necessary because all collation behavior is handled through
-	 * pg_locale_t.
-	 */
+	if (pg_perm_setlocale(LC_COLLATE, collate) == NULL)
+		ereport(FATAL,
+				(errmsg("database locale is incompatible with operating system"),
+				 errdetail("The database was initialized with LC_COLLATE \"%s\", "
+						   " which is not recognized by setlocale().", collate),
+				 errhint("Recreate the database with another locale or install the missing locale.")));
 
 	if (pg_perm_setlocale(LC_CTYPE, ctype) == NULL)
 		ereport(FATAL,