CrashReporter: fix deadlocks by making it mostly async-signal-safe (#5771)

`CrashReporter::createAndSaveCrash()` is not async-signal-safe,
resulting in random deadlocks/double-crashes during Hyprland crashes.
This changes the function to be (mostly) async-signal-safe.

src/signal-safe.hpp

@@ -0,0 +1,172 @@
+#pragma once
+
+#include "defines.hpp"
+
+template <uint16_t N>
+class MaxLengthCString {
+  public:
+    MaxLengthCString() : m_strPos(0), m_boundsExceeded(false) {
+        m_str[0] = '\0';
+    }
+    inline void operator+=(char const* rhs) {
+        write(rhs, strlen(rhs));
+    }
+    void write(char const* data, size_t len) {
+        if (m_boundsExceeded || m_strPos + len >= N) {
+            m_boundsExceeded = true;
+            return;
+        }
+        memcpy(m_str + m_strPos, data, len);
+        m_strPos += len;
+        m_str[m_strPos] = '\0';
+    }
+    void write(char c) {
+        if (m_boundsExceeded || m_strPos + 1 >= N) {
+            m_boundsExceeded = true;
+            return;
+        }
+        m_str[m_strPos] = c;
+        m_strPos++;
+    }
+    void write_num(size_t num) {
+        size_t d = 1;
+        while (num / 10 >= d)
+            d *= 10;
+        while (num > 0) {
+            char c = '0' + (num / d);
+            write(c);
+            num %= d;
+            d /= 10;
+        }
+    }
+    char const* get_str() {
+        return m_str;
+    };
+    bool boundsExceeded() {
+        return m_boundsExceeded;
+    };
+
+  private:
+    char   m_str[N];
+    size_t m_strPos;
+    bool   m_boundsExceeded;
+};
+
+template <uint16_t BUFSIZE>
+class BufFileWriter {
+  public:
+    inline BufFileWriter(int fd_) : m_writeBufPos(0), m_fd(fd_) {}
+    ~BufFileWriter() {
+        flush();
+    }
+    void write(char const* data, size_t len) {
+        while (len > 0) {
+            size_t to_add = std::min(len, (size_t)BUFSIZE - m_writeBufPos);
+            memcpy(m_writeBuf + m_writeBufPos, data, to_add);
+            data += to_add;
+            len -= to_add;
+            m_writeBufPos += to_add;
+            if (m_writeBufPos == BUFSIZE)
+                flush();
+        }
+    }
+    inline void write(char c) {
+        if (m_writeBufPos == BUFSIZE)
+            flush();
+        m_writeBuf[m_writeBufPos] = c;
+        m_writeBufPos++;
+    }
+    inline void operator+=(char const* str) {
+        write(str, strlen(str));
+    }
+    inline void operator+=(std::string_view str) {
+        write(str.data(), str.size());
+    }
+    inline void operator+=(char c) {
+        write(c);
+    }
+    void writeNum(size_t num) {
+        size_t d = 1;
+        while (num / 10 >= d)
+            d *= 10;
+        while (num > 0) {
+            char c = '0' + (num / d);
+            write(c);
+            num %= d;
+            d /= 10;
+        }
+    }
+    void writeCmdOutput(const char* cmd) {
+        int pipefd[2];
+        if (pipe(pipefd) < 0) {
+            *this += "<pipe(pipefd) failed with";
+            writeNum(errno);
+            *this += ">\n";
+            return;
+        }
+        // terminate child instead of waiting
+        {
+            struct sigaction act;
+            act.sa_handler = SIG_DFL;
+            sigemptyset(&act.sa_mask);
+            act.sa_flags    = SA_NOCLDWAIT;
+            act.sa_restorer = NULL;
+            sigaction(SIGCHLD, &act, NULL);
+        }
+        pid_t pid = fork();
+        if (pid < 0) {
+            *this += "<fork() failed with ";
+            writeNum(errno);
+            *this += ">\n";
+            return;
+        }
+        if (pid == 0) {
+            close(pipefd[0]);
+            dup2(pipefd[1], STDOUT_FILENO);
+            char const* const argv[] = {"/bin/sh", "-c", cmd};
+            execv("/bin/sh", (char* const*)argv);
+
+            BufFileWriter<64> failmsg(pipefd[1]);
+            failmsg += "<execv(";
+            failmsg += cmd;
+            failmsg += ") resulted in errno ";
+            failmsg.write(errno);
+            failmsg += ">\n";
+            close(pipefd[1]);
+            abort();
+        } else {
+            close(pipefd[1]);
+            int  len;
+            char readbuf[256];
+            while ((len = read(pipefd[0], readbuf, 256)) > 0) {
+                write(readbuf, len);
+            }
+            if (len < 0) {
+                *this += "<interrupted, read() resulted in errno ";
+                writeNum(errno);
+                *this += ">\n";
+            }
+            close(pipefd[0]);
+        }
+    }
+    void flush() {
+        size_t i = 0;
+        while (i < m_writeBufPos) {
+            int written = ::write(m_fd, m_writeBuf + i, m_writeBufPos - i);
+            if (written <= 0) {
+                return;
+            }
+            i += written;
+        }
+        m_writeBufPos = 0;
+    }
+
+  private:
+    char   m_writeBuf[BUFSIZE];
+    size_t m_writeBufPos;
+    int    m_fd;
+};
+
+char const* sig_getenv(char const* name);
+
+char const* sig_strsignal(int sig);