From ef02e2d5ef63fac8195006e65d006a7b2e6fcd08 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=89rico=20Rolim?= <erico.erc@gmail.com>
Date: Thu, 1 Oct 2020 03:12:02 -0300
Subject: [PATCH] Add pager implementation, use in gemi.

This also required a small change in where gemi outputs the page's
contents, to avoid requiring multiple wait_for_pager() calls in the
code. This also allows it to show the redirect page's contents.
---
 gemi.c   | 36 ++++++++++++++++++++++++-------
 makefile |  2 +-
 pager.c  | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 pager.h  | 17 +++++++++++++++
 4 files changed, 111 insertions(+), 9 deletions(-)
 create mode 100644 pager.c
 create mode 100644 pager.h

diff --git a/gemi.c b/gemi.c
index 3534a99..39553b6 100644
--- a/gemi.c
+++ b/gemi.c
@@ -8,6 +8,7 @@
 #include "mmap_file.h"
 #include "read_certs.h"
 #include "gemini.h"
+#include "pager.h"
 
 #define GEMINI_REQUEST 1024
 
@@ -18,6 +19,7 @@ static void usage(bool fail)
         "Usage: gemi [options] <url>\n"
         "Options:\n"
         "    -b: browse mode (experimental)\n"
+        "    -p: use pager: value of PAGER (default is less)\n"
         "    -n: don't strip header\n"
         "    -d: debug\n"
         "    -h: show this dialog\n"
@@ -30,15 +32,19 @@ static void usage(bool fail)
 int main(int argc, char **argv)
 {
     int rv = EXIT_FAILURE;
-    bool debug = false, no_strip = false, browse = false;
+    bool browse = false, pager = false;
+    bool debug = false, no_strip = false;
     int redirections = 0, redirections_pos = 0;
 
     int c;
-    while ((c = getopt(argc, argv, "+bndhr:")) != -1) {
+    while ((c = getopt(argc, argv, "+bpndhr:")) != -1) {
         switch (c) {
             case 'b':
                 browse = true;
                 break;
+            case 'p':
+                pager = true;
+                break;
             case 'n':
                 no_strip = true;
                 break;
@@ -123,12 +129,20 @@ int main(int argc, char **argv)
 
     signal(SIGPIPE, SIG_IGN);
 
-    // writes directly into stdout
+    FILE *output_stream = stdout;
+    struct pager_proc pager_info;
+    if (pager) {
+        if (launch_pager(&pager_info) < 0) {
+            return rv;
+        }
+        output_stream = pager_info.file;
+    }
+
     struct mmap_file output;
     if (browse) {
         output = create_mmap_from_file(NULL, PROT_MEM);
     } else {
-        output = create_mmap_from_FILE(stdout, "w");
+        output = create_mmap_from_FILE(output_stream, "w");
     }
     if (ERROR_MMAP(output)) {
         return rv;
@@ -151,6 +165,16 @@ int main(int argc, char **argv)
         goto early_out;
     }
 
+    // generic way of outputting data:
+    // - if using FILE backend, offset is 0 and nothing happens
+    // - if using memory backend, offset is used
+    // data output should happen _before_ dealing with links
+    fwrite(output.data, 1, output.offset, output_stream);
+    // pager must always be closed before exec'ing into self
+    // if redirect_link exists, should also kill pager
+    // XXX: add some flag to not kill pager when performing redirection?
+    if (pager) wait_for_pager(pager_info, (bool)redirect_link);
+
     if (redirect_link) {
         redirections += 1;
         // redirect link was stored in callback
@@ -182,10 +206,6 @@ int main(int argc, char **argv)
         execvp(progpath, new_argv);
     }
 
-    // generic way of outputting data:
-    // - if using FILE backend, offset is 0 and nothing happens
-    // - if using memory backend, offset is used
-    fwrite(output.data, 1, output.offset, stdout);
     if (browse) {
         struct gemini_link_node *head = NULL;
         int n = get_links_from_gmi((char *)output.data, &head);
diff --git a/makefile b/makefile
index b4edbcf..1fc2096 100644
--- a/makefile
+++ b/makefile
@@ -12,7 +12,7 @@ INC += -Iextern
 
 BASEENCODEOBJS = extern/libbaseencode/base64.o extern/libbaseencode/base32.o
 PURROBJS = socket.o urls.o files.o comm.o formats.o encrypt.o mmap_file.o
-PURROBJS += read_certs.o gemini.o
+PURROBJS += read_certs.o gemini.o pager.o
 LIBSOBJS = $(BASEENCODEOBJS) $(PURROBJS)
 
 HEADERS = purr.h mmap_file.h read_certs.h gemini.h
diff --git a/pager.c b/pager.c
new file mode 100644
index 0000000..682e8a8
--- /dev/null
+++ b/pager.c
@@ -0,0 +1,65 @@
+#define _POSIX_C_SOURCE 200112L /* fdopen */
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <signal.h>
+#include <sys/wait.h>
+
+#include "pager.h"
+
+int launch_pager(struct pager_proc *p)
+{
+    int rv = -1;
+    int pipes[2];
+
+    if (pipe(pipes) < 0) {
+        perror("pipe()");
+        return rv;
+    }
+
+    char *pager = getenv("PAGER");
+    if (pager == NULL || *pager == 0) {
+        pager = "less";
+    }
+    char *const pager_cmd[] = {pager, NULL};
+
+    p->pid = fork();
+    if (p->pid < 0) {
+        perror("fork()");
+        return rv;
+    }
+
+    if (p->pid == 0) {
+        close(pipes[1]);
+        dup2(pipes[0], STDIN_FILENO);
+        if (execvp(pager, pager_cmd) < 0) {
+            perror("execvp()");
+            return rv;
+        }
+    } else {
+        close(pipes[0]);
+        p->file = fdopen(pipes[1], "w");
+        if (p->file == NULL) {
+            perror("fdopen()");
+            return rv;
+        }
+        rv = 0;
+    }
+
+    return rv;
+}
+
+int wait_for_pager(struct pager_proc p, bool should_kill)
+{
+    int status = 0;
+    fclose(p.file);
+
+    if (should_kill) {
+        sleep(1);
+        kill(p.pid, SIGTERM);
+    }
+
+    waitpid(p.pid, &status, 0);
+
+    return status;
+}
diff --git a/pager.h b/pager.h
new file mode 100644
index 0000000..c6405f9
--- /dev/null
+++ b/pager.h
@@ -0,0 +1,17 @@
+#ifndef __PAGER_H_
+#define __PAGER_H_
+
+#include <stdio.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <sys/types.h>
+
+struct pager_proc {
+    FILE *file;
+    pid_t pid;
+};
+
+int launch_pager(struct pager_proc *);
+int wait_for_pager(struct pager_proc, bool);
+
+#endif // __PAGER_H_