Add page about using Qt pretty printers with GDB

Setup-GDB-with-Qt-pretty-printers.md

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+# Introduction
+
+When debugging qBittorrent with GDB, values of variables with Qt types such as `QByteArray` and  `QString` are not easily printable.
+
+Fortunately, GDB has a pretty printer functionality that enables it to easily print such values in human readable form.
+
+This guide assumes you are relatively inexperienced with GDB, and either don't know what a `.gdbinit` file is or don't use one. If you are an experienced user with an existing `.gdbinit` setup, you should be able follow along and customize your existing setup as needed.
+
+# Setup
+
+1. Clone the qBittorrent repository if you haven't done so already. In this example, it is assumed it was cloned to `/home/user/Documents/qBittorrent`
+
+2. Create a file called `.gdbinit` inside the cloned repository folder with the following contents:
+    ```python
+    python
+
+    import sys, os
+
+    print(f".gdbinit Python: current working directory is {os.getcwd()}")
+    print(f".gdbinit Python: adding custom pretty-printers directory to the GDB path: {os.getcwd() + '/.gdb/qt5prettyprinters'}")
+
+    sys.path.insert(0, "./.gdb/qt5prettyprinters")
+
+    from qt import register_qt_printers
+    register_qt_printers (None)
+
+    end
+    ```
+
+3. Create a folder called `.gdb` inside the cloned repository folder.
+
+4. Download `qt.py` and `helper.py` from [here](https://cgit.kde.org/kdevelop.git/tree/plugins/gdb/printers) and place them inside the `.gdb` folder.
+
+# Usage
+
+After starting a debugging session with qBittorrent (`gdb qbittorrent`, for example), do `source .gdbinit` in  the GDB console. You should see some output informing you that the pretty printers have been loaded.
+
+```gdb
+(gdb) source .gdbinit
+.gdbinit Python: current working directory is /home/user/Documents/qBittorrent
+.gdbinit Python: adding custom pretty-printers directory to the GDB path: /home/user/Documents/qBittorrent/.gdb/qt5prettyprinters
+```
+
+Now, you can use GDB as you usually would. Whenever you print a variable with a Qt type, the output is immediately human-readable without additional effort.
+
+For example, suppose you are at a breakpoint and print the value of a variable named `data`, which is a `QByteArray`, containing an HTTP request.
+
+- output Without pretty printers:
+    ```
+    (gdb) print data
+    $1 = (const QByteArray &) @0x555555e2ecd0: {d = 0x555555e46610}
+    ```
+
+- output with pretty printers:
+
+    ```
+    (gdb) print data
+    $1 = "POST /api/v2/auth/login HTTP/1.1\r\nHost: localhost:8080\r\nUser-Agent: python-requests/2.22.0\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nConnection: keep-alive\r\nContent-Length: 34\r\nContent-Type: appli"... = {[0] = 80 'P', [1] = 79 'O', 
+  [2] = 83 'S', [3] = 84 'T', [4] = 32 ' ', [5] = 47 '/', [6] = 97 'a', [7] = 112 'p', [8] = 105 'i', [9] = 47 '/', [10] = 118 'v', [11] = 50 '2', [12] = 47 '/', [13] = 97 'a', [14] = 117 'u', [15] = 116 't', [16] = 104 'h', [17] = 47 '/', [18] = 108 'l', 
+  [19] = 111 'o', [20] = 103 'g', [21] = 105 'i', [22] = 110 'n', [23] = 32 ' ', [24] = 72 'H', [25] = 84 'T', [26] = 84 'T', [27] = 80 'P', [28] = 47 '/', [29] = 49 '1', [30] = 46 '.', [31] = 49 '1', [32] = 13 '\r', [33] = 10 '\n', [34] = 72 'H', 
+  [35] = 111 'o', [36] = 115 's', [37] = 116 't', [38] = 58 ':', [39] = 32 ' ', [40] = 108 'l', [41] = 111 'o', [42] = 99 'c', [43] = 97 'a', [44] = 108 'l', [45] = 104 'h', [46] = 111 'o', [47] = 115 's', [48] = 116 't', [49] = 58 ':', [50] = 56 '8', 
+  [51] = 48 '0', [52] = 56 '8', [53] = 48 '0', [54] = 13 '\r', [55] = 10 '\n', [56] = 85 'U', [57] = 115 's', [58] = 101 'e', [59] = 114 'r', [60] = 45 '-', [61] = 65 'A', [62] = 103 'g', [63] = 101 'e', [64] = 110 'n', [65] = 116 't', [66] = 58 ':', 
+  [67] = 32 ' ', [68] = 112 'p', [69] = 121 'y', [70] = 116 't', [71] = 104 'h', [72] = 111 'o', [73] = 110 'n', [74] = 45 '-', [75] = 114 'r', [76] = 101 'e', [77] = 113 'q', [78] = 117 'u', [79] = 101 'e', [80] = 115 's', [81] = 116 't', [82] = 115 's', 
+  [83] = 47 '/', [84] = 50 '2', [85] = 46 '.', [86] = 50 '2', [87] = 50 '2', [88] = 46 '.', [89] = 48 '0', [90] = 13 '\r', [91] = 10 '\n', [92] = 65 'A', [93] = 99 'c', [94] = 99 'c', [95] = 101 'e', [96] = 112 'p', [97] = 116 't', [98] = 45 '-', [99] = 69 'E', 
+  [100] = 110 'n', [101] = 99 'c', [102] = 111 'o', [103] = 100 'd', [104] = 105 'i', [105] = 110 'n', [106] = 103 'g', [107] = 58 ':', [108] = 32 ' ', [109] = 103 'g', [110] = 122 'z', [111] = 105 'i', [112] = 112 'p', [113] = 44 ',', [114] = 32 ' ', 
+  [115] = 100 'd', [116] = 101 'e', [117] = 102 'f', [118] = 108 'l', [119] = 97 'a', [120] = 116 't', [121] = 101 'e', [122] = 13 '\r', [123] = 10 '\n', [124] = 65 'A', [125] = 99 'c', [126] = 99 'c', [127] = 101 'e', [128] = 112 'p', [129] = 116 't', 
+  [130] = 58 ':', [131] = 32 ' ', [132] = 42 '*', [133] = 47 '/', [134] = 42 '*', [135] = 13 '\r', [136] = 10 '\n', [137] = 67 'C', [138] = 111 'o', [139] = 110 'n', [140] = 110 'n', [141] = 101 'e', [142] = 99 'c', [143] = 116 't', [144] = 105 'i', 
+  [145] = 111 'o', [146] = 110 'n', [147] = 58 ':', [148] = 32 ' ', [149] = 107 'k', [150] = 101 'e', [151] = 101 'e', [152] = 112 'p', [153] = 45 '-', [154] = 97 'a', [155] = 108 'l', [156] = 105 'i', [157] = 118 'v', [158] = 101 'e', [159] = 13 '\r', 
+  [160] = 10 '\n', [161] = 67 'C', [162] = 111 'o', [163] = 110 'n', [164] = 116 't', [165] = 101 'e', [166] = 110 'n', [167] = 116 't', [168] = 45 '-', [169] = 76 'L', [170] = 101 'e', [171] = 110 'n', [172] = 103 'g', [173] = 116 't', [174] = 104 'h', 
+  [175] = 58 ':', [176] = 32 ' ', [177] = 51 '3', [178] = 52 '4', [179] = 13 '\r', [180] = 10 '\n', [181] = 67 'C', [182] = 111 'o', [183] = 110 'n', [184] = 116 't', [185] = 101 'e', [186] = 110 'n', [187] = 116 't', [188] = 45 '-', [189] = 84 'T', 
+  [190] = 121 'y', [191] = 112 'p', [192] = 101 'e', [193] = 58 ':', [194] = 32 ' ', [195] = 97 'a', [196] = 112 'p', [197] = 112 'p', [198] = 108 'l', [199] = 105 'i'...}
+```
+
+The pretty printer output is further customizable with the usual GDB options. For example, you can suppress output of individual array elements, or reduce the number of array elements printed.