Handle empty or all-blank PAGER setting more sanely in psql.

If the PAGER environment variable is set but contains an empty string,
psql would pass it to "sh" which would silently exit, causing whatever
query output we were printing to vanish entirely.  This is quite
mystifying; it took a long time for us to figure out that this was the
cause of Joseph Brenner's trouble report.  Rather than allowing that
to happen, we should treat this as another way to specify "no pager".
(We could alternatively treat it as selecting the default pager, but
it seems more likely that the former is what the user meant to achieve
by setting PAGER this way.)

Nonempty, but all-white-space, PAGER values have the same behavior, and
it's pretty easy to test for that, so let's handle that case the same way.

Most other cases of faulty PAGER values will result in the shell printing
some kind of complaint to stderr, which should be enough to diagnose the
problem, so we don't need to work harder than this.  (Note that there's
been an intentional decision not to be very chatty about apparent failure
returns from the pager process, since that may happen if, eg, the user
quits the pager with control-C or some such.  I'd just as soon not start
splitting hairs about which exit codes might merit making our own report.)

libpq's old PQprint() function was already on board with ignoring empty
PAGER values, but for consistency, make it ignore all-white-space values
as well.

It's been like this a long time, so back-patch to all supported branches.

Discussion: https://postgr.es/m/CAFfgvXWLOE2novHzYjmQK8-J6TmHz42G8f3X0SORM44+stUGmw@mail.gmail.com

3 files changed, 11 insertions, 3 deletions

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index 168a295013f..0c846e14522 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -3798,8 +3798,9 @@ $endif
       If the query results do not fit on the screen, they are piped
       through this command.  Typical values are
       <literal>more</literal> or <literal>less</literal>.  The default
-      is platform-dependent.  The use of the pager can be disabled by
-      using the <command>\pset</command> command.
+      is platform-dependent.  Use of the pager can be disabled by setting
+      <envar>PAGER</envar> to empty, or by using pager-related options of
+      the <command>\pset</command> command.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/fe_utils/print.c b/src/fe_utils/print.c
index 1ec74f17907..5c5d285be5f 100644
--- a/src/fe_utils/print.c
+++ b/src/fe_utils/print.c
@@ -2874,6 +2874,12 @@ PageOutput(int lines, const printTableOpt *topt)
 			pagerprog = getenv("PAGER");
 			if (!pagerprog)
 				pagerprog = DEFAULT_PAGER;
+			else
+			{
+				/* if PAGER is empty or all-white-space, don't use pager */
+				if (strspn(pagerprog, " \t\r\n") == strlen(pagerprog))
+					return stdout;
+			}
 			disable_sigpipe_trap();
 			pagerpipe = popen(pagerprog, "w");
 			if (pagerpipe)
diff --git a/src/interfaces/libpq/fe-print.c b/src/interfaces/libpq/fe-print.c
index c33dc42a839..e596a514087 100644
--- a/src/interfaces/libpq/fe-print.c
+++ b/src/interfaces/libpq/fe-print.c
@@ -166,8 +166,9 @@ PQprint(FILE *fout, const PGresult *res, const PQprintOpt *po)
 			screen_size.ws_col = 80;
 #endif
 			pagerenv = getenv("PAGER");
+			/* if PAGER is unset, empty or all-white-space, don't use pager */
 			if (pagerenv != NULL &&
-				pagerenv[0] != '\0' &&
+				strspn(pagerenv, " \t\r\n") != strlen(pagerenv) &&
 				!po->html3 &&
 				((po->expanded &&
 				  nTups * (nFields + 1) >= screen_size.ws_row) ||