server/client: add support for sending SIGUSR to footclient

This patch adds the IPC infrastructure necessary to propagate
SIGUSR1/SIGUSR2 from a footclient process to the server process.

By targeting a particular footclient instance, only that particular
instance changes theme. This is different from when targeting the
server process, where all instances change theme.

Closes #2156

CHANGELOG.md

@@ -74,6 +74,10 @@
 * Sending SIGUSR1/SIGUSR2 to a `foot --server` process now causes
   newly spawned client instances to use the selected theme, instead of
   the original one.
+* SIGUSR1/SIGUSR2 can now be sent to `footclient` processes, to change
+  the theme of that particular instance ([#2156][2156]).
+
+[2156]: https://codeberg.org/dnkl/foot/issues/2156
 
 
 ### Deprecated

client-protocol.h

@@ -29,3 +29,17 @@ struct client_data {
 } __attribute__((packed));
 
 _Static_assert(sizeof(struct client_data) == 10, "protocol struct size error");
+
+enum client_ipc_code {
+    FOOT_IPC_SIGUSR,
+};
+
+struct client_ipc_hdr {
+    enum client_ipc_code ipc_code;
+    uint8_t size;
+} __attribute__((packed));
+
+
+struct client_ipc_sigusr {
+    int signo;
+} __attribute__((packed));

client.c

@@ -33,13 +33,20 @@ struct string {
 typedef tll(struct string) string_list_t;
 
 static volatile sig_atomic_t aborted = 0;
+static volatile sig_atomic_t sigusr = 0;
 
 static void
-sig_handler(int signo)
+sigint_handler(int signo)
 {
     aborted = 1;
 }
 
+static void
+sigusr_handler(int signo)
+{
+    sigusr = signo;
+}
+
 static ssize_t
 sendall(int sock, const void *_buf, size_t len)
 {
@@ -507,15 +514,63 @@ main(int argc, char *const *argv)
     if (!send_string_list(fd, &envp))
         goto err;
 
-    struct sigaction sa = {.sa_handler = &sig_handler};
-    sigemptyset(&sa.sa_mask);
-    if (sigaction(SIGINT, &sa, NULL) < 0 || sigaction(SIGTERM, &sa, NULL) < 0) {
+    struct sigaction sa_int = {.sa_handler = &sigint_handler};
+    struct sigaction sa_usr = {.sa_handler = &sigusr_handler};
+    sigemptyset(&sa_int.sa_mask);
+    sigemptyset(&sa_usr.sa_mask);
+
+    if (sigaction(SIGINT, &sa_int, NULL) < 0 ||
+        sigaction(SIGTERM, &sa_int, NULL) < 0 ||
+        sigaction(SIGUSR1, &sa_usr, NULL) < 0 ||
+        sigaction(SIGUSR2, &sa_usr, NULL) < 0)
+    {
         LOG_ERRNO("failed to register signal handlers");
         goto err;
     }
 
     int exit_code;
-    ssize_t rcvd = recv(fd, &exit_code, sizeof(exit_code), 0);
+    ssize_t rcvd = -1;
+
+    while (true) {
+        rcvd = recv(fd, &exit_code, sizeof(exit_code), 0);
+
+        const int got_sigusr = sigusr;
+        sigusr = 0;
+
+        if (rcvd < 0 && errno == EINTR) {
+            if (aborted)
+                break;
+            else if (got_sigusr != 0) {
+                LOG_DBG("sending sigusr %d to server", got_sigusr);
+
+                struct {
+                    struct client_ipc_hdr hdr;
+                    struct client_ipc_sigusr sigusr;
+                } ipc = {
+                    .hdr = {
+                        .ipc_code = FOOT_IPC_SIGUSR,
+                        .size = sizeof(struct client_ipc_sigusr),
+                    },
+                    .sigusr = {
+                        .signo = got_sigusr,
+                    },
+                };
+
+                ssize_t count = send(fd, &ipc, sizeof(ipc), 0);
+                if (count < 0) {
+                    LOG_ERRNO("failed to send SIGUSR IPC to server");
+                    goto err;
+                } else if ((size_t)count != sizeof(ipc)) {
+                    LOG_ERR("failed to send SIGUSR IPC to server");
+                    goto err;
+                }
+            }
+
+            continue;
+        }
+
+        break;
+    }
 
     if (rcvd == -1 && errno == EINTR)
         xassert(aborted);

doc/foot.1.scd

@@ -694,14 +694,14 @@ variables to unset may be defined in *foot.ini*(5).
 The following signals have special meaning in foot:
 
 - SIGUSR1: switch to color theme 1 (i.e. use the *[colors]* section).
-- SIGUSR2: switch to color theme 2 (i.e. use the *[colors2]* section)-
+- SIGUSR2: switch to color theme 2 (i.e. use the *[colors2]* section).
 
 Note: you can send SIGUSR1/SIGUSR2 to a *foot --server* process too,
 in which case all client instances will switch theme. Furthermore, all
 future client instances will also use the selected theme.
 
-Sending SIGUSR1/SIGUSR2 to a footclient instance is currently not
-supported.
+You can also send SIGUSR1/SIGUSR2 to a footclient instance, see
+*footclient*(1) for details.
 
 
 # BUGS

doc/footclient.1.scd

@@ -189,6 +189,21 @@ variables may be defined in *foot.ini*(5).
 In addition to the variables listed above, custom environment
 variables to unset may be defined in *foot.ini*(5).
 
+# Signals
+
+The following signals have special meaning in footclient:
+
+- SIGUSR1: switch to color theme 1 (i.e. use the *[colors]* section).
+- SIGUSR2: switch to color theme 2 (i.e. use the *[colors2]* section).
+
+When sending SIGUSR1/SIGUSR2 to a footclient instance, the theme is
+changed in that instance only. This is different from when you send
+SIGUSR1/SIGUSR2 to the server process, where all instances change the
+theme.
+
+Note: for obvious reasons, this is not supported when footclient is
+started with *--no-wait*.
+
 # SEE ALSO
 
 *foot*(1)

server.c

@@ -156,10 +156,61 @@ fdm_client(struct fdm *fdm, int fd, int events, void *data)
     xassert(events & EPOLLIN);
 
     if (client->instance != NULL) {
-        uint8_t dummy[128];
-        ssize_t count = read(fd, dummy, sizeof(dummy));
-        LOG_WARN("client unexpectedly sent %zd bytes", count);
-        return true;  /* TODO: shutdown instead? */
+        struct client_ipc_hdr ipc_hdr;
+        ssize_t count = read(fd, &ipc_hdr, sizeof(ipc_hdr));
+
+        if (count != sizeof(ipc_hdr)) {
+            LOG_WARN("client unexpectedly sent %zd bytes", count);
+            return true;  /* TODO: shutdown instead? */
+        }
+
+        switch (ipc_hdr.ipc_code) {
+        case FOOT_IPC_SIGUSR: {
+            xassert(ipc_hdr.size == sizeof(struct client_ipc_sigusr));
+
+            struct client_ipc_sigusr sigusr;
+            count = read(fd, &sigusr, sizeof(sigusr));
+            if (count < 0) {
+                LOG_ERRNO("failed to read SIGUSR IPC data from client");
+                return true; /* TODO: shutdown instead? */
+            }
+
+            if ((size_t)count != sizeof(sigusr)) {
+                LOG_ERR("failed to read SIGUSR IPC data from client");
+                return true; /* TODO: shutdown instead? */
+            }
+
+            switch (sigusr.signo) {
+            case SIGUSR1:
+                term_theme_switch_to_1(client->instance->terminal);
+                break;
+
+            case SIGUSR2:
+                term_theme_switch_to_2(client->instance->terminal);
+                break;
+
+            default:
+                LOG_ERR(
+                    "client sent bad SIGUSR number: %d "
+                    "(expected SIGUSR1=%d or SIGUSR2=%d)",
+                    sigusr.signo, SIGUSR1, SIGUSR2);
+                break;
+            }
+
+            return true;
+        }
+
+        default:
+            LOG_WARN(
+                "client sent unrecognized IPC (0x%04x), ignoring %hhu bytes",
+                ipc_hdr.ipc_code, ipc_hdr.size);
+
+            /* TODO: slightly broken, since not all data is guaranteed
+               to be readable yet */
+            uint8_t dummy[ipc_hdr.size];
+            read(fd, dummy, ipc_hdr.size);
+            return true;
+        }
     }
 
     if (client->buffer.data == NULL) {