  This is the first paper in a series of studies of the Coma cluster using the
\textit{SRG}/eROSITA X-ray data obtained in course of the Calibration and
Performance Verification observations. The data cover $\sim3^\circ\times
3^\circ$ area around the cluster with a typical exposure time of more than 20
ks. The stability of the instrumental background and operation of the
\textit{SRG} Observatory in the scanning mode provided us with an excellent
data set for studies of the diffuse emission up to a distance of $\sim
1.5R_{200}$ from the Coma center. In this study, we discuss the rich morphology
revealed by the X-ray observations (also in combination with the SZ data) and
argue that the most salient features can be naturally explained by a recent
(on-going) merger with the NGC 4839 group. In particular, we identify a faint
X-ray bridge connecting the group with the cluster, which is convincing proof
that NGC 4839 has already crossed the main cluster. The gas in the Coma core
went through two shocks, first through the shock driven by NGC 4839 during its
first passage through the cluster some Gyr ago, and, more recently, through the
"mini-accretion shock" associated with the gas settling back to
quasi-hydrostatic equilibrium in the core. After passing through the primary
shock, the gas should spend much of the time in a rarefaction region, where
radiative losses of electrons are small, until the gas is compressed again by
the mini-accretion shock. Unlike "runway" merger shocks, the mini-accretion
shock does not feature a rarefaction region downstream and, therefore, the
radio emission can survive longer. Such a two-stage process might explain the
formation of the radio halo in the Coma cluster.
